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THE 

CHEMISTRY  OF  CYANIDE  SOLUTIONS 

RESULTING  FROM 

THE  TREATMENT  OF  ORES. 


BY 
J.  E.  CLENNELL,  B.  Sc.  (LOND.) 

Associate  of  the  Institution  of  Mining  &  Metallurgy. 

Author  of  'Analytical  Work  in  Connection  with  the  Cyanide 
Process/ 


NEW  YORK  AND  LONDON 

THE  ENGINEERING  AND  MINING  JOURNAL 
1904 


Copyright,  1904, 

BY 
THE  ENGINEERING  AND  MINING  JOURNAL. 


TN 


C5<?c 


CONTENTS. 

-» 

PAGE 

:£  INTRODUCTORY 1 

\jr-  INGREDIENTS  OF  CYANIDE  SOLUTIONS  THAT  ARE  ESTIMATED.  .  2 

^  ACTIVE  CYANOGEN  COMPOUNDS 4 

><           Free  Cyanide 4 

§           Total  Cyanide 33 

CQ           Total  Cyanogen 47 

Hydrocyanic  Acid 52 

Available  Cyanide 54 

ALKALINE  CONSTITUENTS 58 

Total  Alkali 62 

^           Protective  Alkali 63 

Hydrates,  Carbonates  and  Bicarbonates 66 

Ammonia    68 

H  REDUCING  AGENTS 70 

Reducing  Power  70 

<L           Ferrocyanides 73 

Thiocyanates 87 

Sulphides 90 

Other  Reducing  Agents 93 

>„  AUXILIARY  AGENTS 95 

3          Oxygen 95 

^          Active  Haloids . . 100 

*          Peroxides 101 

Ferricyanides 102 

'•->  INACTIVE  BODIES 104 

Cyanates  and  Isocyanates 108 

Chlorides 112 

Nitrates 112 

Sulphates  112 

Silicates   .  113 


IV  CONTENTS. 

PAGE 

NOBLE  METALS 114 

Gold  and  Silver  Together 114 

Gold  Alone ., 118 

Silver  Alone ....  120 

BASE  METALS ,,....,... 123 

Zinc 123 

Copper 123 

Iron,  Alkaline  Earths  and  Alkali  Metals 137 

SUSPENDED  MATTER 138 

Total   Solids  in   Suspension 138 

Various  Constituents  in  Suspended  Matter 140 

Total  Solids  in  Solution 141 

AN  EXAMINATION  OF  VARIOUS  METHODS  FOR  THE  ESTIMATION 

OF  FERROCYANIDE 143 


INTRODUCTORY. 


In  preparing  the  following  treatise,  my  object  has  been  not  so 
much  to  give  the  results  of  any  special  researches  on  individual 
obscure  points  as  to  present  a  comprehensive  and,  so  far  as  possi- 
ble, complete  review  of  the  entire  subject.  For  this  purpose  a  short 
description  of  well-known  methods  is  introduced,  and,  where  neces- 
sary, a  critical  discussion  of  their  value.  I  have  also  described  the 
various  modifications  of  existing  methods  that  have  been  suggested 
from  time  to  time,  but  which  have  not  hitherto  been  collected  and 
compared,  and  have  given  the  results  of  experiments  made  to  test 
the  accuracy  of  the  assumptions  on  which  such  modifications  are 
based.  While,  for  the  sake  of  completeness  and  the  clear  presenta- 
tion of  the  subject,  it  has  been  necessary  to  include  much  that  is 
already  familiar,  it  is  hoped  that  the  points  discussed  are  shown  to 
be  of  sufficient  interest  and  importance  to  justify  a  somewhat  ex- 
tended investigation. 

A  systematic  study  of  the  solutions  resulting  from  the  continued 
working  of  the  cyanide  process  on  some  particular  class  of  ore  may 
throw  much  light  on  the  chemical  and  economic  problems  involved 
in  the  treatment,  and  in  some  instances  has  proved  of  great  prac- 
tical value.  It  is  highly  desirable,  therefore,  to  have  a  fairly 
simple,  rapid  and  reliable  system  of  laboratory  tests  for  determin- 
ing the  amount  of  any  of  the  more  important  constituents  of  such 
solutions.  In  addition  to  these  laboratory  methods,  one  or  two 
rough  tests  are  needed  which  will  suffice  for  controlling  the  daily 
routine  operations  of  the  plant;  such  tests  should  give  a  clear  and 
unmistakable  indication,  and  should  represent  some  factor  of  real 
value  in  the  treatment,  though  strict  scientific  accuracy  is  not  a  neces- 
sity in  this  case. 


NOTE. — The  author,  owing  to  absence  on  professional  work,  was  unable  to 
revise  his  proofs.  The  revision  was  done  by  Mr.  H.  E.  Bowles,  F.I.C.,  to  whom 
acknowledgment  is  due  by  the  publishers  for  this  courtesy. 


3  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

INGREDIENTS   OF   CYANIDE   SOLUTIONS   THAT   ARE 
ESTIMATED. 

For  the  purpose  of  analysis,  the  various  constitutents  of  a  cyanide 
solution,  after  use  in  the  treatment  of  ores,  may  be  conveniently 
classified  as  follows:  — 

Class  I.  —  Active  Cyanogen  Compounds. 

Class  II.  —  Alkaline  Constituents. 

Class  III.  —  Reducing  Agents. 

Class  IV.  —  Auxiliary  Agents. 

Class  V.  —  Inactive  Bodies. 

Class  VI.—  Noble  Metals. 

Class  VII.—  Base  Metals. 

Class  VIII.—  Suspended  Matter. 

The  estimations  which  will  be  considered  under  each  of  these 
heads  are  here  summarized. 

CLASS  I.  —  ACTIVE  CYANOGEN  COMPOUNDS. 


1.  Free  cyanide.  %•  Total  cyanogen. 

J  3  2.  Total  cyanide.  4.  Hydrocyanic  acid. 

5.  'Available'  cyanide. 

CLASS  II.—  ALKALINE  CONSTITUENTS. 

1.  Total  alkali.  -    4.  Alkaline   carbonates    and   bicarbon* 

2.  'Protective'  alkali.  ates. 

3.  Alkaline  hydrates.      5.  Ammonia  and  ammonium  salts. 

CLASS  III.—  REDUCING  AGENTS. 

1.  Reducing  power.  4.  Organic  matter. 

2.  Ferrocyanides.  5.  Sulphides. 

3.  Thiocyanates.  6.  Nitrites. 

CLASS  IV.  —  AUXILIABY  AGENTS. 

1.  Oxygen.  3.  Peroxides. 

3.  Active  haloids.  4.  Ferricyanides. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

CLASS  V. — INACTIVE  BODIES. 

1.  Cyanates  and  isocyanates.  3.  Nitrates. 

2.  Chlorides.  4.  Sulphates. 

5.  Silicates. 

CiAss  VI. — NOBLE  METALS. 

1.  Gold  and  silver  together.  2.  Gold  alone. 

3.  Silver  alone. 

CLASS  VII. — BASE  METALS. 

1.  Zinc.  3.  Iron. 

2.  Copper.  4.  Alkaline  earths. 

5.  Alkali  metals. 

CLASS  VIII. — INSOLUBLE  (SUSPENDED)  MATTEB. 
(Organic  and  Inorganic.) 

In  exceptional  cases  some  accidental  impurities  not  included  in 
the  above  list  may  have  to  be  estimated.  Thus  organic  matter  has 
been  supposed  to  exist  in  these  solutions  in  a  variety  of  forms,  some 
of  considerable  complexity,  but  as  a  rule  no  useful  object  would  be 
served  by  special  estimations  of  these  bodies,  e.  g.,  formates,  oxam- 
ides,  urea,  etc.,  and  they  will  not  be  considered  in  the  present  discus- 
sion. :  - 


4  OHEMISTEY    OF    CYANIDE    SOLUTIONS. 

CLASS    I. 

ACTIVE  CYANOGEN  COMPOUNDS. 

Under  this  head  will  be  considered  those  cyanogen  compounds 
which  are  of  more  or  less  practical  value  as  solvents  for  the  precious 
metals.  For  the  sake  of  convenience  we  shall  also  discuss  the  estima- 
tion of  certain  factors  ('total  cyanide'  and  'total  cyanogen'),  of 
cyanogen  compounds  that  are  not  solvents  of  gold  and  silver,  in  addi- 
tion to  those  which  are  solvents. 


SECTION  1. 
ESTIMATION  OF  FREE  CYANIDE. 

The  term  'free  cyanide'  will  be  used  to  indicate  the  equivalent, 
in  terms  of  potassium  cyanide,  of  all  the  cyanogen  which  is  present 
in  the  solution  as  simple  cyanides  of  the  alkalis  and  alkaline  earth 
metals,  such  as  KCy,  NaCy,  NH4Cy,  CaCy2  and  BaCy2.  It  doea 
not  include  cyanogen  present  in  the  form  of  double  cyanides  or 
hydrocyanic  acid. 

METHODS. 

The  principal  methods  hitherto  proposed  for  estimating  free 
cyanide  are  as  follows : 

1.  The  silver  nitrate  method,  proposed  in  its  original  form  by 
Liebig. 

2.  The  iodine  method,  generally  ascribed  to  Fordos  and  Gelis. 

3.  The  mercuric  chloride  method  of  Hannay. 

4.  The  cuprammonium  method. 

METHOD  No.  1. 

Estimation  of  Free  Cyanide  by  Titration  with  Silver  Nitrate. 
(A)     Liebig's  Method  (Unmodified), 

This  method,  either  in  its  original  form  or  with  certain  modi- 
fications to  be  discussed  later,  is  the  one  almost  universally  adopted 
in  the  actual  daily  tests  made  for  regulating  the  strength  of  solu- 
tions in  use  for  the  treatment  of  ores. 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  5 

The  process  depends  upon  the  fact  that  when  a  solution  of 
nitrate  of  silver  is  added  drop  by  drop  to  a  liquid  containing  simple 
cyanides  of  the  alkali  or  alkaline  earth  metals,  each  drop  of  the 
solution  forms  a  white  cloud  of  silver  cyanide.  So  long  as  the  free 
cyanide  is  in  excess,  this  cloud  disappears  on  agitation,  forming 
a  soluble  double  cyanide  of  silver  by  combination  with  the  simple 
cyanide  present.  The  reactions  in  the  case  of  potassium  cyanide 
are  as  follows: 

(a)  AgN08  +  KCy  =  AgCy  +  KN08. 


The  completion  of  the  reaction  is  shown  by  the  permanence  of  a 
white  turbidity  or  opalescence.  When  this  point  is  reached  the 
whole  of  the  free  cyanide  has  been  converted  into  a  double  salt  of 
silver,  and  any  further  addition  of  silver  nitrate  will  cause  a  pre- 
cipitate of  silver  cyanide  which  does  not  redissolve;  thus: 

(c)  AgN03  +  KAgCy2  =  2AgCy  +  KN03. 

It  will  be  noted  from  these  reactions  that  one  molecule  of  silver 
nitrate  is  equivalent  to  two  molecules  of  potassium  cyanide,  the 
complete  series  of  reactions  up  to  the  point  where  a  permanent  tur- 
bidity appears  being  expressible  in  one  equation,  as  follows  : 

(d)  AgN03  +  2KCy  =  KAgCy2  +  KN03. 

[Hence  169.55  parts  of  AgN"03  are  equivalent  to  130.04  parts 
of  KCy,  or  1.3038  parts  AgN03  =  1  part  of  KCy.] 

'Standard  Solutions.  —  The  solution  most  commonly  used  in  test- 
ing by  this  method  is  prepared  by  dissolving  13.038  grams  of  pure 
crystallized  nitrate  of  silver  in  distilled  water,  and  diluting  until 
the  whole  volume  of  the  solution  is  1,000  c.c.  Every  c.c.  of  this  solu- 
tion is  equivalent  to  0.01  gram  KCy.  Hence  if  we  take  10  c.c.  of 
the  cyanide  solution  which  is  to  be  tested,  every  c.c.  of  the  silver 
solution  added  will  represent  0.1  per  cent,  of  'free  cyanide.' 

For  testing  dilute  solutions,  such  as  are  commonly  used  at  the 
present  day  in  ore  treatment,  it  will  be  more  convenient  to  prepare 
a  standard  silver  nitrate  solution  of  half  this  strength,  i.e.,  con- 
taining 6.519  grams  of  silver  nitrate  dissolved  to  a  liter.  Every 
c.c.  of  this  solution  is  equivalent  to  0.005  gram  KCy;  hence  if 
we  take  50  c.c.  (a  convenient  quantity  for  dilute  solutions)  of  the 
liquid  to  be  tested,  every  c.c.  of  the  standard  AgN08  added  will 
represent  0.01  per  cent  KCy. 

If  a  decinormal  solution  of  silver  nitrate,  however,  be  used, 


6  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

1  c.c.  =  0.013004  gram  KCy;  hence  if  we  take  13  c.c.  of  the 
liquid  to  be  tested,  1  c.c.  N/10  AgN03=:0.1  per  cent  free  cyanide. 

Mode  of  Carrying  Out  the  Test. — A  measured  volume  (varying, 
according  to  the  strength  of  the  solution  to  be  tested,  from  5  to 
100  c.c.)  is  taken  by  means  of  a  measuring  column  or  pipette,  and 
transferred  to  a  small  flask  or  beaker,  a  small  conical  Erlenmeyer 
flask  being  very  suitable  for  the  titration.  The  silver  nitrate  solu- 
tion is  now  run  in  from  a  burette.  If  the  strength  of  cyanide  is 
approximately  known,  the  standard  solution  may  be  added  rapidly 
at  first,  finishing  drop  by  drop.  The  flask  should  be  placed  in  a 
good  light  and  the  reaction  observed  against  a  dark  background. 
The  foot  of  the  burette-stand  may  be  painted  a  dull  black  or  well 
rubbed  with  charcoal.  It  is  sometimes  found  convenient  to  have  the 
burette-stand  raised  until  the  flask  containing  the  solution  to  be 
tested  is  about  on  a  level  with  the  eye,  so  that  the  finishing  point 
may  be  conveniently  observed  by  looking  through  the  liquid.  With 
pure  solutions  no  difficulty  will  be  found  in  observing  the  exact 
point  at  which  the  solution  in  the  flask  becomes  permanently 
turbid. 

Very  strong  solutions,  such  as  contain  1  per  cent  KCy  or  over, 
should  be  diluted  with  distilled  water  before  testing,  as  otherwise  the 
silver  cyanide  may  be  precipitated  in  a  granular  form  which  dissolves 
with  difficulty  in  the  excess  of  cyanide.  (In  some  cases  this  granu- 
lar precipitate  seems  to  consist  wholly  or  partially  of  carbonate  of 
silver.)  Any  difficulty  in  observing  the  finishing  point  due  to  this 
cause  may  generally  be  obviated  by  the  use  of  the  alkaline  iodide 
indicator  to  be  described  below. 

Solid  Cyanide. — In  testing  samples  of  solid  cyanide  a  certain 
quantity  (say  1  to  5  grams)  is  weighed  out  exactly  in  a  stoppered 
bottle,  or  weighing  tube,  and  dissolved  in  a  large  quantity  of  dis- 
tilled water.  The  solution  is  then  made  up  to  a  definite  volume, 
say,  a  liter,  and  after  thorough  mixing,  a  measured  portion  (50  or 
100  c.c.)  is  tested,  as  above  described,  with  standard  silver  nitrate 
solution.  In  this  connection  it  is  well  to  bear  in  mind  that  what 
is  determined  by  this  and  other  methods  of  estimating  free  cya- 
nide is  not  actually  the  potassium  cyanide,  but  the  cyanogen, 
which  may  in  reality  be  present  as  a  sodium,  calcium,  ammonium 
or  other  salt.  This  cyanogen  being  reported  as  its  equivalent  in 
potassium  cyanide,  i  sample  of  pure  cyanide  of  sodium  would 
appear  to  contain  over  132  per  cent  KCy. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  7 

Turbid  Solutions. — When  the  solutions  are  at  all  turbid,  they 
must  be  filtered,  as  the  end-point  cannot  be  observed  with  any  ap- 
proach to  accuracy  unless  the  liquid  be  perfectly  clear.  Solutions 
occurring  in  ore  treatment  sometimes  contain  very  finely  divided 
matter  in  suspension,  which  will  pass  through  all  ordinary  filter 
papers,  and  cannot  be  removed  even  by  repeated  filtration.  In  such 
cases  the  liquid  may  generally  be  clarified,  however,  by  the  addi- 
tion of  lime,  which  causes  a  flocculation  and  settlement  of  the  sus- 
pended matter,  and  allows  the  liquid  to  be  filtered  perfectly  clear. 
With  solutions  free  from  zinc,  and  otherwise  comparatively  pure, 
it  is  necessary,  after  lime  has  been  used  for  the  above  purpose,  to 
add  the  potassium  iodide  indicator  described  below,  when  a  per- 
fectly correct  titration  of  the  free  cyanide  is  required.  When  zinc 
is  present,  the  addition  of  lime  is  inadmissible ;  the  effect  produced 
will  be  discussed  later  under  'Estimation  of  Total  Cyanide.' 

Influence  of  Foreign  Salts  on  Liebig's  Method. 

/  The  method  above  detailed  works  admirably  with  pure  cyanide 
solutions,  but  gives  very  uncertain  and  inaccurate  results  in  pres- 
ence of  some  of  the  impurities  which  are  generally  introduced 
during  treatment  of  ores. 

Zinc  occurs  chiefly  as  double  cyanide  of  zinc  and  potassium, 
perhaps  also  as  a  soluble  double  ferrocyanide  of  the  same  metals. 
The  finishing  point  is  almost  always  obscure  and  indefinite,  and 
is,  moreover,  affected  by  a  variety  of  circumstances,  which  will  be 
discussed  in  detail  later  on. 

Alkalis  (hydrates,  carbonates  and  ammonium  salts)  cause  slight 
errors,  the  apparent  strength  being  generally  in  excess  of  the  truth. 
With  free  ammonia  the  error  is  considerable. 

Ferrocyanides,  thiocyanates  and  chlorides  slightly  interfere,  also 
rendering  the  results  too  high,  but  the  error  is  scarcely  appreciable 
unless  excessive  quantities  are  present. 

Thiosulphates  (hyposulphites)  render  the  titration  quite  errone- 
ous, as  they  readily  dissolve  the  cyanide  of  silver,  so  that  the  results 
appear  much  higher  than  the  normal. 

Sulphides  give  an  immediate  black  or  brown  coloration,  which 
obscures  and  entirely  vitiates  the  result  of  the  titration  with  silver. 
The  method  of  procedure  when  sulphides  are  present  will  be  detailed 
later. 


8  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

(4J     Liebig's  Method  with  Sodium  Chloride  as  Indicator. 

In  Liebig's.original  paper  (Ann.  der  Chem.  u.  Pharm.,  77,  102) 
the  addition  of  sodium  chloride  is  recommended  as  an  indicator,  ap- 
parently on  the  assumption  that  a  precipitate  of  silver  chloride  will 
occur  immediately  on  adding  a  drop  of  silver  nitrate  in  excess  of  the 
amount  required  to  convert  all  the  cyanide  into  KAgCy,  (or  an 
equivalent  compound) .  It  has  been  shown,  however,  by  Gr.  Deniges 
'  (Ann.  de  Chim.  et  de  P/flH<«4,,  Series  7,  Vol.  VI.,  p.  381)  that  chlo- 
™  rides  cannot  be  precipitated  by  silver  salts  in  presence  of  cyanides 
such  as  KCy,  etc.,  until  after  the  complete  precipitation  of  the 
cyanides  as  AgCy.  Since  the  same  reactions  take  place  whether 
sodium  chloride  be  present  or  not,  it  is  evidently  of  no  value  as  an 
indicator. 

(B)    Liebig's  Method  with  the  Potassium  Iodide  Indicator. 
Neutral  Iodide  Indicator. 

The  use  of  potassium  iodide  as  an  indicator  for  the  finishing  point 
of  the  reaction  in  Liebig's  method  is  stated  to  have  been  suggested 
originally  by  J.  T.  McArthur,  and  depends  on  the  following  con- 
siderations : 

(a)  Iodide  of  silver  is  readily  dissolved  by  free  alkaline  cyanides. 

(&)  Iodide  of  silver  is  almost  absolutely  insoluble  in  solutions 
of  alkaline  hydrates,  monocarbonates  and  ammonium  salts,  and 
also  in  free  ammonia,  which  latter  dissolves  cyanide  and  chloride  of 
silver  easily. 

(c)  In  a  mixture  containing  cyanides,  chlorides  and  iodides  in 
solution,  the  iodide  will  be  precipitated  in  preference  to  the  cyanide, 
and  the  cyanide  in  preference  to  the  chloride,  on  addition  of  silver 
nitrate. 

(d)  The  presence  of  a  small  trace  of  iodide  of  silver  imparts  a 
yellowish  tinge  to  the  precipitate,  which  makes  the  exact  finishing 
point  somewhat  more  distinct. 

When  the  iodide  indicator  is  used,  the  successive  reactions  occur- 
ring on  addition  of  silver  nitrate  appear  to  be  as  follows: 

Momentary  formation  of  silver  iodide,  where  AgN03  is  locally  in 

(a)  Agff03  +  KI  =  Agl  +  KNO,. 


CHEMISTRY    OF    CYANIDE     SOLUTIONS.  9 

This  dissolves  on  agitation,  so  long  as  the  liquid  contains  an  excess 
of  cyanide: 

(6)  Agl  +  2KCy  =  KI  +  KAgCy2. 

Precipitation  of  silver  iodide,  when  no  more  free  cyanide  is  pres- 
ent, the  precipitate  remaining  permanent  on  agitation,  and  showing 
a  faint  yellow  tinge : 

(c)  AgN03  +  KI  =  Agl  +  KN03. 

This  latter  reaction  occurs  in  preference  to  the  reaction 

AgN03  -f  KAgCy2  =  2AgCy  -f  KN03 
so  long  as  potassium  iodide  is  present. 

G.  Deniges  (Ann.  de  Chim.  et  de  Phti/mL,  Series  7,  Vol.  VI., 
p.  381)  describes  the  following  experiment,  which  throws  consider- 
able light  on  the  reactions  of  silver  with  cyanides  and  haloid  salts : 

Twenty  c.c.  of  1  per  cent  potassium  cyanide  were  taken  in  each 
case  and  mixed  with 

(a)  0.5  gram  sodium  chloride. 

(&)  0.5  gram  potassium  bromide. 

(c)  0.5  gram  potassium  iodide. 

Sufficient  silver  nitrate  was  added  to  each  to  give  a  slight  precipitate. 
This  was  dissolved  by  1  c.c.  of  the  cyanide  solution,  and  then  silver 
nitrate  solution  again  added  in  quantity  equal  to  that  originally 
used.  The  precipitate,  after  washing,  was  examined  in  each  case, 
and  showed  the  following  composition: 

(a)  Exclusively  silver  cyanide. 

(6)  Silver  cyanide  with  a  very  little  silver  bromide. 

(c)  Exclusively  silver  iodide. 

It  will  be  seen  that  the  use  of  this  indicator  corrects  any  error 
that  might  arise  from  the  presence  of  alkali  or  ammonia.  It  will 
also  be  evident  that  chlorides  or  bromides  would  be  useless  as  indi- 
cators. 

The  test  is  generally  made  by  adding  a  few  drops  of  a  strong  (say 
10  per  cent)  solution  of  potassium  iodide  to  the  liquid  which  is  to 
be  titrated.  I  have  generally  found  it  more  convenient  to  add  to 
each  test  from  5  to  10  c.c.  of  a  neutral  1  per  cent  solution  of  po- 
tassium iodide. 

Correction  of  Errors. — Any  errors  introduced  in  the  titration  as 
made  without  indicator,  owing  to  the  presence  of  alkalis,  ammonia, 
ammonium  salts,  chlorides,  ferrocyanides  or  thiocyanates,  are  more 
or  less  corrected  by  the  addition  of  iodide  indicator.  (See  tables  of 
results.) 


10  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

Titration  in  Presence  of  Zinc. — In  solutions  containing  little  or  no 
free  alkali,  a  whitish  turbidity  (ZnCy2)  forms  at  a  certain  stage, 
and  increases  on  addition  of  more  AgN03.  The  yellow  precipitate 
of  Agl,  momentarily  formed,  is  dissolved  on  agitation  until  a  further 
point  is  reached,  at  which  a  faint  yellow  tint  becomes  permanent. 
In  exactly  neutralized  solutions  (i.e.,  solutions  containing  no  alka- 
line hydrates  or  monocarbonates),  the  first  appearance  of  a  distinct 
whitish  turbidity  indicates  the  free  cyanide.  [In  strongly  alkaline 
solutions  the  white  precipitate  either  does  not  remain  permanent 
on  agitation  or  only  forms  towards  the  end  of  the  titration,  and  the 
final  yellow  tint  indicates  the  so-called  total  cyanide,  i.e.,  the  equiva- 
lent, in  terms  of  KCy,  of  the  cyanogen  present  as  simple  cyanides, 
together  with  that  as  K2ZnCy4.  This  matter  will  be  fully  treated 
under  'total  cyanide.'] 

The  method  of  obtaining  an  exactly  neutralized  solution  will  be 
given  in  discussing  the  determination  of  'protective  alkali/ 

Titration  in  Presence  of  Copper. — When  the  solution  contains  a 
double  cyanide  of  copper  and  an  alkali  metal,  the  indication  ob- 
tained with  the  iodide  indicator  is  much  lower  than  without  it. 
The  number  obtained  with  the  iodide  appears  to  correspond  with 
the  actual  free  cyanide  present.  Without  the  indicator  a  portion  of 
the  cyanogen  of  the  double  salt  is  also  determined.  This  case  may 
also  be  more  conveniently  treated  under  'total  cyanide.' 

Alkaline  Iodide  Indicator. — The  addition  of  a  strong  solution  of 
caustic  potash  or  soda,  together  with  potassium  iodide,  has  been  in 
general  use  for  some  time  past  for  estimating  'total  cyanide'  and 
appears  to  have  been  independently  suggested  by  Bettel  and  Goyder 
in  1895.  The  reactions  will  be  discussed  under  'total  cyanide.' 

G-.  Denig&s  in  1893  published  a  method  of  estimating  silver  by 
adding  an  excess  of  standard  potassium  cyanide,  and  determining 
the  excess  by  titrating  with  N/10  silver  nitrate,  with  addition  of 
ammonia  and  potassium  iodide,  until  a  faint  cloud  of  silver  iodide 
is  formed.  In  a  further  investigation  published  in  1897  (Ann.  de 
Chim.  et  de  Pharm.,  Series  7,  Vol.  VI.,  p.  381),  he  recommends  this 
method  for  the  estimation  of  cyanogen,  stating  that  the  finishing 
point  is  much  sharper  than  that  obtained  in  Liebig's  original 
process,  or  even  than  that  obtained  by  the  use  of  neutral  potassium 
iodide  as  indicator.  The  granular  precipitate  sometimes  observed 
in  titrating  cyanide  solutions,  especially  towards  the  finish,  does  noi 
occur,  or  is  rapidly  redissolved  in  presence  of  ammonia. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  11 

W.  J.  Sharwood  (Journal  Amer.  Chem.  Soc.,  1897,  p.  400)  gives 
a  detailed  account  of  the  application  of  this  process  in  the  titration 
of  impure  solutions,  and  recommends  as  indicator  5  c.c.  of  commer- 
cial ammonia  with  2  c.c.  of  5  per  cent  potassium  iodide,  to  be  added 
to  each  test,  taking  25  to  100  c.c.  of  the  cyanide  solution  to  be  ex- 
amined. For  smaller  quantities  of  cyanide  solution  he  uses  1  c.c. 
each  of  ammonia  and  iodide.  The  indicator  may,  of  course,  be 
made  up  in  a  single  solution. 

I  generally  use  an  indicator  made  by  dissolving  10  grams  of 
potassium  iodide  in  500  c.c.  of  ordinary  ammonia  solution,  and 
diluting  to  1,000  c.c.  For  each  test  5  to  10  c.c.  of  this  indicator 
are  added  to  50  c.c.  of  the  liquid  to  be  titrated. 

In  presence  of  zinc  this  method  serves  practically  for  the  determi- 
nation of  'total  cyanide/  Sharwood  states,  however,  that  the  am- 
moniacal  iodide  indicator  always  gives  somewhat  low  results  in  the 
titration  of  solutions  containing  K2ZnCy4,  unless  caustic  alkali  be 
added  as  well. 

The  alkali  or  ammoniacal  iodide  indicators  also  correct  any  errors 
due  to  ferrocyanides,  sulphocyanides,  chlorides,  hydrates  and 
ammonium  salts.  The  error  introduced  by  the  presence  of  thio- 
eulphates  is  much  diminished,  but  does  not  appear  to  be  entirely 
eliminated.  The  extent  of  these  errors,  and  the  use  of  the  indicator 
in  correcting  them  is  clearly  shown  in  the  following  tables. 


TESTS  ILLUSTRATING  THE  INFLUENCE  OF  ALKALIS  ON  THE  NITRA- 
TION OF  CYANIDE  WITH  SILVER  NITRATE,  AND  CORRECTION 
OF  THE  ERROR  BY  POTASSIUM  IODIDE  INDICATOR. 

No.  1. — HYDRATES. 

Solutions  used. 

(a)  Pure  cyanide,  0.47%  KCy,  containing  alkali  equivalent  to 
0.006%  NaOH. 

(&)  Sodium  hydrate,  alkalinity  to  phenol  phthalein  equivalent 
to  3.84%  NaOH. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgN03.    1  c.c.  =  0.005  gram  KCy. 


12 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


Details  of  Test. 

Volume  of  cyanide  solution  (a)  in  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.047  gram. 


Volume 
of  NaOH 
sol.  (b) 
added. 

Volume 
of  KI 
sol.  (c) 
added. 

Volume 

Of  AgNOa 
sol.  (d) 
required. 

Weight 
of  NaOH 
in  portion 
tested. 

Weight 
of  KCy 
indi- 
cated. 

Cyanide 
Indicated 
in  original 
solution. 

c.c. 

c.c. 

<r.c. 

Grams. 

Grams. 

% 

Without 

9  40 

0  0006 

0  047.0 

0  470 

iodide. 

5 

9.55 

0.1926 

0.0477 

0.477 

50 

9  90 

0  3846 

0  0495 

0  495 

5 

1010 

0  5766 

0  0505 

0  505 

25 

10.35 

0.9606 

0  0517 

0  517 

With 

1 

9.40 

0.0006 

0.0470 

0.470 

iodide. 

y 

5 

m 

0.5766 
0.9606 

0.0470 
0.0475 

0.470 
0.475 

25 

5 

9.40 

0.9606 

0.0470 

0.470 

No.  2. — AMMONIA. 

Solutions  used. 

(a)  Pure  cyanide,  0,47%  KCy,  containing  alkali  equivalent  to 
0.006%  NaOH. 

(b)  Ammonia  (10%  of  strong  solution),  10  c.c.  of  solution  (&) 
required  155  c.c  N/10  acid  with  methyl  orange,  indicating  2.635% 
NH3. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgN03;  1  c.c.  =  0.005  gram  KCy. 

Details  of  Test. 

Volume  of  cyanide  solution  (a)  in  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.047  gram. 


Volume 
of  NH. 
sol.  (b) 
added. 

C.C. 

Volume 
of  KI 
sol.  (c) 
added. 

C.C. 

Volume 
of  ApNO, 
sol.  (d) 
required. 

C.C. 

Weight 
of  NH, 
in  portion 
tested. 

Grams. 

Weight 
of  KCy 
indi- 
cated. 

Grams. 

Cyanide 
indicated 
in  original 
solution. 

% 

Without 

940 

0  0470 

0.470 

iodide. 

1 

1030 

0.026 

0.0515 

0.515 

4 



14.80 

0.132 
0263 

0.0740 
7 

0.740 
? 

With 

1 

9  40 

00470 

0.470 

Iodide. 

10 
10 
25 

3 
6 

9.50 
9.35 
9.45 

0.263 
0.263 
0.659 

0.0475 
0.0467 
0.0472 

0.475 
0.467 
0.472 

CHEMISTRY    OF    CYANIDE    SOLUTIONS.  13 

The  error  (in  absence  of  iodide)  is  thus  seen  to  be  much  more 
serious  with  ammonia  than  with  sodium  hydrate. 

No.  3.— SODIUM  CARBONATE. 
Solutions  used. 

(a)  Pure  cyanide,  0.465%  KCy. 

(&)  Sodium  carbonate,  10%  (from  anhydrous  Na2C03). 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgN03;'l  c.c.  =  0.005  gram  KCy. 

Details  of  Test. 

Volume  of  cyanide  solution  (a)  in  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.0465  gram. 


• 

Volume 
of 
Na,CO» 
sol.  (It) 
added. 

Volume 
of 
KI 
sol.  (c) 
added. 

Volume 
of 
AgNO. 
sol  (d) 
required. 

Weight 

Na°2CO» 
in  portion 
tested. 

Weight 

III 

cated. 

Cyanide 
indicated 
in  original 
solution. 

c.c. 

c.c. 

c.c. 

Grams. 

Grams. 

% 

Without 

9.30 

0  0465 

0.465 

Iodide 

10 

9  25 

1 

0  0462 

0  462 

20 

9  30 

2 

0  0465 

0  465 

With 
Iodide. 

"20"" 

1 
1 

9.30 
9.25 

'"2"' 

0.0465 
0.0462 

0.465 
0.462 

The  above  results  show  that  no  appreciable  error  was  introduced, 
even  without  using  the  iodide  indicator,  by  the  presence  of  2  grams 
of  sodium  carbonate.  The  finishing  point  in  absence  of  KI  was, 
however,  rather  indistinct,  and  in  all  cases  (with  or  without  KI) 
a  slight  whitish  turbidity  occurred  just  before  the  finish. 

Tests  in  which  sodium  hydrate  or  ammonia  were  added,  as  well 
as  sodium  carbonate,  gave  the  correct  finishing  point  when  titrated 
with  AgNO 3  in  presence  of  KI. 


No.  4.— SODIUM  CARBONATE. 
Solutions  used. 

(a)  Potassium  cyanide,  0.715%  KCy. 
(6)  Sodium  carbonate,  3.78%  Na2C08. 


t  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgN08  (1  c.c.  =  0.005  gram  KCy). 


Details  of  Test. 

Volume  of  cyanide  solution  (a)  in  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.0715  gram. 


Volume 
of 
Na2Co» 
sol.  (6) 
taken. 

c.c. 

Volume 
of 
water 
added. 

C.C. 

Volume 
of 
neutral 
1%  KI 
added. 

c.c. 

Volume 

AgNOi 
sol.  (d) 
required. 

C.C. 

Weight 

Na°CO, 

In  portion 
tested. 

Grama. 

Weight 

KCy 
Indi- 
cated. 

Grams. 

Cyanide 
Indicated 
in  original 
solution. 

With- 

14.45 

0.0722 

0.722 

50 

144 

0  0720 

0  720 

Iodide. 

25 

14.4 

0.044 

0.0720 

0.720 

50 

14.5 

1.888 

0.0725 

0.725 

With 

5 

14.4 

0.0720 

0.720 

Iodide 

50 

14.3 

0  0715 

0  715 

""56"' 

14  3 

1  888 

0  0715 

0  715 

solid 

50 

5 

14.35 

1.000 

0.0717 

0.717 

The  solution  gave  a  slight  turbidity  before  the  proper  finishing 
point,  but  otherwise  there  was  no  evidence  of  any  interference 
owing  to  presence  of  sodium  carbonate. 

The  apparent  strength  of  cyanide  was  slightly  diminished  by 
dilution. 

No.  5. — AMMONIUM  CARBONATE. 
Solutions  used. 


(a)  Pure  cyanide,  0.47%  KCy. 

(6)  Ammonium  carbonate,   showing  by  titration  with   N/10 
acid  and  methyl  orange,  6.288%   (NH4)2C03. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgN03;  1  c.c.  =  0.005  gr.  KCy. 

Details  of  Test. 

Volume  of  cyanide  solution  (a)  in  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.047  gram. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


15 


Volume 
of  (NH«), 
CO. 
sol.  (6) 
added. 

c.c. 

Volume 
of 
KIsol.(c) 
added. 

c.c. 

Volume 
of 
Ag  NO, 
sol.  (d) 
required. 

c.c. 

Weight 
of  (NH<)» 
C0»  in 
portion 
tested. 

Grams. 

W$tt 

KCy 
indi- 
cated. 

Grams. 

Cyanide 
indicated 
in  original 
solution. 

% 

Without 

9  40 

0.0470 

0.470 

iodide. 

1 

9.45 

0.063 

0.0472 

0.472 

5 

10  20 

0  314 

0  0510 

0  510 

10 

11  10 

0.629 

0  0555 

0.555 

15 

11  65 

0943 

00582 

0  582 

With 

3 

9  35 

0  0467 

0  46T 

iodide. 

10 
15 

1 
8 

9.20 
9.30 

0.620 
0.943 

0.0460 
0.0465 

0.460 
0.465 

Here  the  results  in  presence  of  iodide  show  a  tendency  to  appear 
slightly  lower  than  the  truth. 


No.  6. — SODIUM  BICABBONATE. 

Solutions  used. 

(a)  Potassium  cyanide,  0.47%  KCy  (containing  a  trace  of 
alkali). 

(&)  Sodium  bicarbonate,  prepared  from  commercial  salt. 
Combined  titrations  of  this  solution  with  decinormal  acid,  using 
phenol  phthalein  and  afterwards  methyl  orange,  indicated 
1.42%  Na2C03 
1.78%  NaHC03. 

(c)  Potassium  iodide  (neutral),  1%. 

(d)  Silver  nitrate,  0.652%  AgN08;  1  c.c.  =  0.005  gram  KCy. 

(e)  Nitric  acid,  N/10,  standardized  with  pure  sodium  carbonate 
and  methyl  orange. 

In  some  of  the  following  tests  the  carbonate  shown  to  be  present 
in  solution  (6)  was  neutralized  or  nearly  neutralized  with  N/10 
nitric  acid,  in  accordance  with  the  equation: 

Na2C03  +  HN03  =  NaHC08  -f  NaN08. 


Details  of  Test. 

Volume  of  cyanide  solution  (a)  in  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.047  gram. 


16 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


Volume 
of 

Volume 
of  N/10 

Volume 
of 

Weight 
of  actual 

Weight 
of 

Cyanide 
indicated 

blcarbon- 
atesol.(b) 
added. 

HNO, 
sol.  (e) 
added. 

AgNO, 

sol.  fit 
required. 

NaHCO. 
in  port'n 
tested. 

KCy 
indi- 
cated. 

in  original 
solution. 

c.c. 

c.c. 

c.c. 

Grams. 

Grama. 

% 

940 

0  0470 

0  470 

iodide 

5 

9.40 

0.089 

00470 

0  470 

10 

9  35 

0  178 

0  0467 

0  467 

10 

12.0 

9.25 

0.259 

0.0462 

0.462 

15 

9  45 

0  267 

0  0472 

20.0 

9.35 

0.435 

0.0467 

0.467 

20 

26.8 

9.25 

0.581 

0.0462 

0.462 

Volume 
of  bicarb, 
sol.  (b) 
added. 

Volume 
of  N/10 
HNO, 
sol.  (e) 
added. 

Volume 
of 
KI 
sol.  (c) 
added. 

Volume 
of 
AgNO. 
sol.  (d) 
required. 

Weight 
of  actual 
NaHCO, 
in  port'n 
tested. 

^l!1' 
indi^ 
cated. 

Cyanide 
Indicated 
in  original 
solution. 

c.c. 

c.c. 

c.c. 

c.c. 

c.c. 

Grams. 

% 

With 

j 

9  40 

0  0470 

0  470 

iodide. 

20 

26.8 

1 

9.30 

0.581 

0.0465 

0.465 

In  cases  where  the  monocarbonate  was  exactly  neutralized  the 
silver  precipitate  dissolved  rather  slowly  towards  the  finish,  and 
sometimes  showed  a  tendency  to  assume  a  granular  form. 

The  results  in  presence  of  much  bicarbonate  seem  to  be  slightly 
lower  than  the  truth. 

TESTS  ILLUSTRATING  THE  INFLUENCE  OF  CHLORIDES  ON  THE  Ti- 

TRATION   OF   CYANIDES  WITH    SILVER  NlTRATE. 

No.  1. — SODIUM  CHLORIDE. 


Solutions  used. 

(a)  Potassium  cyanide,  0.4025%  KCy. 

(6)  Sodium  chloride  (pure  crystallized  NaCl). 

(c)  Potassium  iodide,  10%  KI  (neutral). 

(<f)  Silver  nitrate,  0.652%  AgN03;  1  c.c.  =  0.005  gram  KCy. 

Details  of  Test. 

Cyanide  solution  (a)  used  in  each  titration  =  10  c.c. 
Weight  of  KCy  in  portion  tested  =  0.04025  gram. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


1? 


Weight 
of 
NaCl 
crystals 
added. 

Grams. 

Volume 
of 
KI 
sol.  (c) 
added. 

c.c. 

Volume 
of 
AgNO, 
sol.  (d) 
required. 

c.c. 

wsr 

KCy 
indi- 
cated. 

Grams. 

Cyanide 
indicated 
in  original 
solution. 

Percent- 
age of  er- 
ror above 
true 
value. 

% 

Without 

8  10 

0  0405 

0  405 

0  6 

iodide 

0  5 

8  30 

0  0415 

0  415 

3  1 

10 

8  50 

0  0425 

0  425 

5  6 

With 
iodide 

'6  5* 

1 
1 

8.05 
8.05 

0.04025 
0  04025 

0.4025 
0  4025 



1.0 

1 

8.05 

0.04025 

0.4025 

One  gram  of  the  sodium  chloride  (&)  dissolved  in  a  little  water 
gave  an  immediate  precipitate  with  a  single  drop  of  AgN03,  which 
did  not  redissolve  on  agitation;  hence  it  appears  that  the  error  is 
not  due  to  solubility  of  silver  chloride  in  sodium  chloride. 

No.  2. — AMMONIUM  CHLORIDE. 

Solutions  used. 

(a)  Potassium  cyanide,  0.47%  KCy. 

(&)  Ammonium  chloride,  5.34%  NH4C1,  neutral  to  phenol 
phthalei'n  and  methyl  orange. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgN03;  1  c.c.  =  0.005  gram  KCy. 

Details  of  Test. 

Cyanide  solution  (a)  used  in  each  titration  =  10  c.c. 
Weight  of  KCy  in  portion  tested  =  0.047  gram. 


Volume 
of 

Volume 
of 

Volume 
of 

Weight 
of 

Weight 

Cyanide 
indicated 

NH«C1 
sol.  (o) 
added. 

KI 

sol.  (c) 
added. 

AgNOs 

sol.  (d) 
required. 

NH«C1  in 
portion 
tested. 

ind£ 
cated. 

in  original 
solution. 

c.c. 

c.c. 

c.c. 

Grams. 

Grams. 

% 

9.40 

0.0470 

0.470 

iodide 

5 

9.45 

0.267 

0.0472 

0.472 

10 

9.70 

0.534 

0.0485 

0.485 

15 

9.70 

0.801 

0.0485 

0.485 

20 

9  85 

1.068 

0.0492 

0.492 

With 

1 

9.40 

0.0470 

0.470 

iodide. 

5 

1 

8.85 

0.267 

0.0442 

0.442 

10 

1 

8.65 

0.534 

0.0432 

0.432 

15 

1 

8.60 

0.801 

0.0430 

0.430 

20 

1 

8.50 

1.068 

0.0425 

0.425 

15 

3 

8.45 

0.801 

0.0422 

0.422 

20 

3 

8.30 

1.068 

0.0415 

0.415 

20 

5 

8.00 

1.068 

0.0400 

0.400 

18 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


From  these  tests  it  appears  that  the  presence  of  ammonium  chlo- 
ride causes  the  apparent  strength  of  cyanide  to  be  higher  than  the 
truth  when  the  solution  is  titrated  without  indicator.  When  neutral 
KI  is  added,  however,  the  result  is  lower  than  the  truth,  and  the 
solution  shows  a  white  turbidity  (sometimes  rather  indefinite)  at 
the  finish.  With  ammonium  carbonate  the  results  in  presence  of 
KI  were  also  somewhat  lower  than  the  truth,  but  the  error  was  less 
than  in  the  case  of  the  chloride.  Addition  of  sufficient  caustic 
alkali  or  ammonia,  together  with  the  iodide^  corrects  this  error,  as 
seen  in  the  following  tests : 

Cyanide  solution  (a)  used  in  each  case  =  10  c.c. 
Weight  of  KCy  in  portion  tested  =  0.047  gram. 
Volume  of  NH4C1  solution  (&)  in  each  case  —  20  c.c. 
Weight  of  NH4C1  in  portion  tested  =  1.068  grams. 


Volume 

Volume 

Weight 

Cyanide 

of 

of 

of 

Indicated 

Alkali  Added. 

KI 
sol.  (c) 
added. 

AgNO, 
sol.  (d) 
required. 

S 

cated. 

in  original 
solution. 

c.c. 

c.c. 

Grams. 

% 

4  c  c    3  85%  NaOH                  

1 

92 

0  0460 

0  460 

5  c.c.   NH4OH    (10%  of  strong)  

3 

9!l5 
9  4 

0.0457 
0  0470 

0.457 
0  470 

No.  3.  —  AMMONIUM  CHLORIDE. 
Solutions  used. 


(a)  Potassium  cyanide,  0.725%  KCy. 

(&)  Ammonium  chloride,  1%  NH4C1. 

(c)  Potassium  iodide  (neutral),  \%  KI. 

(d)  Potassium  iodide  (alkaline),  1%  KI, 

(e)  Silver  nitrate,  0.652%  AgN03  (1  c.c. 


NaOH. 
0.005  gram  KCy). 


Details  of  Test. 


Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  KCy  in  portion  tested  =  0.0725  gram. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


19 


Volume 
of 
NH4C1 
sol.  (6) 
added. 

c.c. 

Volume  of  Io- 
dide indicator 
added. 

Volume 
of 
AgNO, 
sol.  (e) 
required. 

c.c. 

wept 

NH4C1 
in  portion 
tested. 

Grams. 

Weight 
of  KCy 
indicated. 

Grams. 

Cyanide 
indi- 
cated in 
original 
solution. 

% 

Neu- 
tral, 
(c) 

c.c. 

Alka- 
line, 
(d) 
c.c. 

Without 
iodide. 

14.5 

0.0725 
0.0725 
0.0725 
0.0727 
0.0730 

0.725 
0.725 
0.725 
0.727 
0.730 

15 
80 
45 

65 

14.5 
14.5 
14.55 
14.6 

0.15 
0.80 
0.45 
0.65 

With 
neutral 
iodide. 

5 
5 
5 
5 
5 

14.5 
14.1 
13.9 
13.7 
18.5 

0.0725 
0.0705 
0.0695 
0.0685 
0.0675 

0.725 
0.705 
0.695 
0.685 
0.675 

15 
30 
45 

65 

0.15 
0.30 
0.45 
0.65 



With 
alkaline 
iodide. 

65 
65 

:::::::: 

5 

10 

14.8 
14.5 

0.65 
0.65 

0.0715 
0.0725 

0.715 
0.725 

TESTS  ILLUSTRATING  THE  INFLUENCE  OF   FERROCYANIDES 
SULPHOCYANIDES  ON  THE  TlTRATION  OF  CYANIDES  BY 
SILVER  NITRATE,  AND  CORRECTION  OF  THE  ERROR 
BY  MEANS  OF  POTASSIUM  IODIDE. 


AND 


I. — FERROCYANIDES. 
TEST  No.  1. 

Solutions  used. 

(a)  Cyanide,  0.565%  KCy. 

(&)  Potassium  ferrocyanide  (N/10),  K4FeCy,.3H20  =  4.22%. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  AgN08  1.304%  (1  c.c.  =  0.01  gram  KCy). 

Detaili  of  Test. 

Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  cyanide  in  each  portion  tested  =  0.0565  gram. 


CHEMISTRY   OF   CYANIDE    SOLUTIONS. 


Volume 
of  N/10 
ferro- 
cyanide 

Volume 
of  IX  KI 
sol.  (c) 
added. 

Volume 
of  AgNO, 

80l..(d) 

required. 

Weight 
of  ferro- 
cyanide 
present. 

Weight 
of  KCy 
indicated. 

Cyanide 
indicated 
in  original 
solution. 

added. 

c.c. 

c.c. 

C.C. 

Grams. 

Grams. 

% 

Without 

560 

0  0560 

0.560 

iodide. 

5 

6.50 

0.211 

00650 

0.650 

10 

705 

0  422 

00705 

0-705 

With 

5 

565 

00565 

0.565 

iodide. 

5 

5 

5.65 

0.211 

0.0565 

0.565 

10 

5 

5.65 

0.422 

0.0565 

0.565 

TEST  No.  2. 
Solutions  used. 

(a)  Potassium  cyanide,  0.342%  KCy. 

(6)  Potassium  ferrocyanide,  1%  K4FeCy6.3H20. 

(c)  Potassium  iodide  (alkaline),  1%  KI,  4%  NaOH. 

(d)  Silver  nitrate,  0.652%  AgN03  (1  c.c.  =  0.005  gram  KCy). 


Details  of  Test. 

Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  cyanide  in  each  portion  tested  =  0.0342  gram. 


Volume 

Volume 

Volume 

Weight 

Weight 

Cyanide 

rftt 

ferro- 
cyanide(6) 

of  1* 
KI  sol.  (c) 
added. 

of  AgNO, 
sol.  (d) 
required. 

of  ferro- 
cyanide 
present. 

of  KCy 
indicated. 

indicated 
in  original 
solution. 

added. 

c.c. 

C.C. 

c.c. 

Grams. 

Grams. 

% 

6  85 

0  0342 

iodide. 

5 

745 

005 

0  0372 

0  372 

10 

820 

0  10 

0  0410 

15 

8  50 

0  15 

0  0425 

90 

890 

020 

0  0445 

25 

9  10 

025 

0  0455 

60 

9  25 

0  50 

0  0462 

With 

20 

1 

7.20 

0.20 

0.0360 

0.360 

iodide. 

90 

8 

6.80 

0.20 

0.0340 

0.340 

50 

3 

7.05 

0.50 

0.0352 

0.352 

50 

5 

6.80 

0.50 

0.0340 

0.340 

CHEMISTRY  OF   CYANIDE   SOLUTIONS.  21 

II. — SULPHOCYANIDES    (THIOCYANATES) . 

TEST  No.  1. 

Solutions  used. 

(a)  Potassium  cyanide,  0.4%  KCy. 

(6)  Ammonium  sulphocyanide,  0.5%  NH4CyS. 

(c)  Potassium  iodide  (alkaline),  1%  KI,  4%  NaOH. 

(d)  Silver  nitrate,  AgN03  0.652%  (1  c.c.  =  0.005  gram  KCy). 

Details  of  Test. 

Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.04  gram. 


Volume 
of  .5* 
sulphocy- 
anide (6) 
added. 

c.c. 

Volume 
of  alk. 
KI(c) 
added. 

c.c. 

Volume 
of  AgNO, 
sol.  (d) 
required. 

c.c. 

Weight 
of  sulpho- 
cyamde 
present. 

Grains. 

Weight 
of  KCy 
indicated. 

Grams. 

Cyanide 
indicated 
in  original 
solution. 

% 

Without 

8  0 

0  0400 

0  400 

iodide 

5 

8  0 

0  025 

0  0400 

0  400 

10 

8.1 

0.05 

0.0405 

0.405 

15 

8  05 

0  0402 

0  402 

25 

8.05 

0.125 

0.0402 

0  402 

With  iodide. 

25 

5 

8.0 

0.125 

0.0400 

0.400 

From  this  test  it  appears  that  small  amounts  (up  to  0.1  gram) 
of  ammonium  sulphocyanide  do  not  appreciably  affect  the  result  of 
the  titration. 

TEST  No.  2. 
Solutions  used. 

(a)  Potassium  cyanide,  0.465%  KCy. 

(&)  Ammonium  sulphocyanide   (crystals),  NH4CyS. 

(c)  Potassium  iodide,  (Alkaline  indicator),  KI  1%  ;  NaOH  4%. 

(d)  Silver  nitrate,  0.652%  AgN08  (1  c.c.  =  0.005  gram  KCy). 

Details  of  Test. 

Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.0465  gram. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


Weight 
of 
NH4CyS 
crystals 
added. 

Volume 
of  alk. 
KI  sol.  (c) 
added. 

Volume 
of  AgNO» 
sol.  (d) 
required. 

Weight 
ofKCy 
indicated. 

•Corrected 
weight 
ofKCy. 

Cyanide 

indicated 
in  original 
solution. 

Grama. 

c.c. 

c.c. 

Grams. 

Grams. 

% 

Without 

9  SO 

0  0465 

0  0465 

0  465 

iodide 

1 

15  05 

0  0752 

With 
iodide. 

1 

5 

9.40 

0.0470 

0.0458 

0.458 

•  This  correction  was  made  because  it  was  found  that  1  gram  of  the  NH4CyS  dis- 
solved In  a  little  water,  to  which  5  c.c.  of  the  alkaline  iodide  Indicator  were  added, 
required  0.25  c.e.  of  AgNO,  to  give  a  distinct  turbidity,  this  result  being  possibly 
due  to  the  presence  of  a  trace  of  free  cyanide  in  the  salt  used. 

The  test  shows  that  large  quantities  of  sulphocyanide  may  cause 
a  serious  error,  which,  however,  is  corrected  by  the  alkaline  iodide 
indicator. 

TEST  No.  3. 
Solutions  used. 

(a)  Potassium  cyanide,  0.765%  KCy. 
(&)   Potassium  sulphocyanide,  0.5%  KCyS. 

(c)  Potassium  iodide  (neutral),  1%  KI. 

(d)  Silver  nitrate,  0.652%  AgNOs  (1  c.c.  =  0.005  gram  KCy). 

Details  of  Test. 

Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  cyanide  in  portion  tested  =  0.0765  gram. 


Volume 
of 

Volume 
of 

Volume 
of 

Weight 
of 

Weight 
of 

Cyanide 
indicated 

KCyS 
sol.  (6) 
added! 

neutral 
KI 
added. 

AgNO, 
(d) 
required. 

KCyS 
in  portion 
tested. 

cyanide 
indicated. 

in  original 
solution. 

c.c. 

c.c. 

c.c. 

Grams. 

Grams. 

% 

Without 

15  4 

0  0770 

0  770 

iodide 

10 

16.85 

0.05 

0.767 

20 

15.45 

0.10 

0  0772 

0.772 

80 

15.4 

0.15 

0.0770 

0.770 

-  • 

40 

15.4 

0.20 

0.0770 

0.770 

.  ...  * 

50 

15.35 

0.25 

0.767 

solid 

18  5 

1  00 

0  0925 

0  925 

f  50  c.c. 

solid 

16  7 

1.00 

0.785 

-;  water 

1  added. 

With 

5 

15  8 

0  0765 

0  765 

iodide. 

50 

6 

15.2 

0.25 

0.0760 

0.760 

solid 

5 

15.8 

1.00 

0.0765 

0.765 

CHEMISTRY   OF    CYANIDE    SOLUTIONS. 


TESTS  ILLUSTRATING  THE  INFLUENCE  OF  THIOSULPHATES  (HYPO- 
SULPHITES)  ON  THE  TlTRATION  OF  CYANIDES  WITH 

SILVER  NITRATE. 
Solutions  used. 

(a)  Cyanide,  0.286%. 

(6)  Sodium  thiosulphate  (N/10),  Na2S203.5H20  =  2A8%. 

(c)  Potassium  iodide  (neutral),  \%  KI. 

(d)  Potassium  iodide  (alkaline),  4%  NaOH,  1%  KI. 

(e)  Potassium  iodide  ammoniacal,  \%  KI. 

(/)  Silver  nitrate,  1  c.c.  =  0.005  gram  KCy;  AgN03,  0.652%. 

Details  of  Test. 

Cyanide  solution  (a)  taken  for  each  titration  =  10  c.c. 
Weight  of  cyanide  in  each  portion  tested  =  0.0572  gram. 


Vol.  of 
N/10 

Volume  of 
Iodide  Indicator 

Volume 
of 

Weight 

Weight 
of 

Cyanide 
indicated 

thiosul- 

added. 

AgNO, 

thiosul- 

KCy 

in 

phate 
added. 

iol. 
required. 

phate 
present. 

indi- 
cated. 

original 
solution. 

(c) 

(tf) 

(e) 

Neu- 

Alka- 

Am- 

tral. 

line. 

moni- 

acal. 

c.c. 

c.c. 

C.C. 

C.C. 

C.C. 

Grams. 

Grams. 

Grams. 

Without 

11  5 

0  287 

iodide 

...... 

12.8 

0.0248 

0  0640 

0  320 

5 

19  4 

0  124 

0  0970 

0  485 

10 

29.75 

0.248 

0  1987 

0.744 

With 

t 

11.45 

0  0572 

0  286 

iodide 

10 

5 

12  9 

0  248 

0  0645 

0  322 

10 

10 

18  5 

0  248 

0  0625 

0  312 

10 

5 

12.5 

0.248 

0.0625 

0  812 

10 

10 

12  0 

0  248 

0  0600 

6  300 

10 

15 

12.0 

0.248 

0.0600 

0.300 

20 

10 

12  8 

0  496 

0  0640 

0  320 

10 

10 

12.6 

0.248 

0.0630 

0.315 

(C)     Liebig's  Method  with  Potassium  Ferrocyanide  as  Indicator. 

W.  Bettel  proposes  the  following  method  (Proceedings  Chem. 
and  Met.  Soc.  of  South  Africa,  Vol.  I.,  p.  165,  August  17,  1895)  : 

"Fifty  c.c.  of  solution  is  taken  and  titrated  with  silver  nitrate  to 
faint  opalescence,  (or  first  appearance  of  a  flocculent  precipitate). 
This  will  indicate  (if  sufficient  ferrocyanide  be  present  to  form  a 


24  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

flocculent  precipitate  of  zinc  ferrocyanide)  the  free  cyanide,  and 
cyanide  equal  to  7.9  per  cent,  of  the  potassic  zinc  cyanide. 

W.  J.  Sharwood  (Engineering  and  Mining  Journal,  1898,  p. 
216),  also,  adopts  the  same  process  and  uses  a  few  drops  of  a  5  per 
cent  ferrocyanide  solution  as  indicator. 

The  value  of  this  method,  however,  is  more  than  doubtful,  as 
may  be  judged  from  the  following  remarks  by  Mr.  Bettel  (loc.  citf 
p.  163),  on  the  reactions  taking  place  in  presence  of  the  zinc-double 
cyanide : 

"On  titration  with  nitrate  of  silver  the  end-reaction  is  painfully 

indefinite. If   to  a  solution   of   potassic   zinc   cyanide 

be  added  a  small  quantity  of  ferrocyanide  of  potassium  and  the 
silver  solution  run  in,  the  flocculent  precipitate  of  what  I  suppose  to 
be  normal  zinc  ferrocyanide  (Zn2FeCy6)  appears;  the  end-reaction 
is  fairly  sharp,  and  indicates  19.5  parts  of  cyanide  of  potassium  out 
of  the  actual  molecular  contents  of  130.2  KCy.  If,  however,  an 
excess  of  ferrocyanide  be  present,  the  flocculent  precipitate  does  not 
appear,  but  in  its  place  one  gets  an  opalescence  which  speedily  turns 
to  a  finely  granular  (sometimes  slimy)  precipitate  of  potassic  zinc 
ferrocyanide,  K2Zn3Fe2Cy12.  This  introduces  a  personal  equation 
into  the  analysis  of  such  a  solution,  for,  if  the  silver  solution  be 
added  rapidly,  the  results  are  higher  than  if  added  drop  by  drop, 
as  this  ferrocyanide  of  zinc  and  potassium  separates  out  slowly  in 
dilute  solutions  alkaline  or  neutral  to  litmus  paper." 

The  finishing  point,  in  presence  of  zinc,  is  also  very  seriously 
affected  by  dilution,  as  shown  by  G.  A.  Goyder  (Chemical  News, 
Vol.  LXXIL,  p.  80).  (See  also  below.) 

Bettel's  quantity  ("cyanide  equal  to  7.9  per  cent,  of  the  potassic 
zinc  cyanide  present")   arises  as  follows:     Assuming  247  as  the 
molecular  weight  of  K2ZnCy4,  and  that,  of  this  weight,  19.5  parts 
are  indicated  by  the  test  as  free  KCy,  we  have  the  proportion : 
19.5:  247::  7.9: 100. 

Dr.  T.  Kirke  Rose  (Metallurgy  of  Gold,  4th  ed.,  p.  398),  com- 
menting on  Goyder's  observations  as  to  the  effect  of  dilution,  quoted 
below,  remarks  as  follows:  "Bettel's  amount,  7.9  per  cent  of  the 
double  cyanide,  probably,  therefore,  corresponds  only  to  one  definite 
concentration.  The  double  cyanide  is  doubtless  dissociated  more  or 
less  into  2KCy  -j-  ZnCy2  in  dilute  solutions,  and  the  potassium 
cyanide  set  free  is  perhaps  available,  both  to  dissolve  gold  and  to  keep 
silver  cyanide  remaining  dissolved  in  the  water  in  which  it  is  not 


CHEMISTEY    OF    CYANIDE    SOLUTIONS.  25 

quite  insoluble In  any  case  it  seems  quite  likely  that  the 

free  cyanide  shown  by  simple  tritation  represents  the  cyanide  avail- 
able for  the  dissolution  of  gold,  so  that,  from  the  practical  point  of 
view,  no  new  methods  are  necessary." 

Considering  the  extreme  indefiniteness  of  the  indications  generally 
obtained  in  all  attempts  to  estimate  free  cyanide  in  presence  of 
zinc,  this  last  conclusion  can  hardly  be  accepted. 

(D)  Estimation   of  Free   Cyanide   by   Titration,  as  in  Liebig's 

Method,  after  Neutralizing  Alkali  and  Adding  Potassium 

Iodide. 

In  mixtures  containing  both  zinc  double  cyanide  and  free  cyanide, 
two  points  may  in  general  be  observed  on  titrating  with  silver  nitrate 
in  presence  of  potassium  iodide:  (a)  The  first  appearance  of  a  per- 
manent (flocculent)  whitish  turbidity;  (6)  The  appearance  of  a 
distinct  yellowish  coloration  or  precipitate. 

In  strongly  alkaline  solutions  the  second  point  corresponds,  as 
will  be  shown  later,  with  total  cyanide. 

In  solutions  containing  no  free  alkali,  or  only  moderate  amounts, 
it  is  decidedly  lower  than  the  amount  required  for  total  cyanide. 

In  solutions  containing  no  free  alkali,  the  first  appearance  of  a 
flocculent  precipitate,  in  presence  of  potassium  iodide,  gives  a  read- 
ing which  is,  in  general,  slightly  higher  than  the  theoretical  contents 
of  the  solution  in  free  cyanide.  The  point  is  fairly  definite,  and 
although  not  absolutely  satisfactory,  appears  to  be  the  most  reliable 
method  so  far  described  for  determining  this  quantity  in  presence 
of  zinc.  It  is  possible  that  an  actual  dissociation  of  the  K2ZnCy4 
into  KCy  and  ZnCy2  may  take  place  to  some  extent,  and  that  the 
test  correctly  indicates  the  free  cyanide.  In  some  solutions  the 
numbers  obtained  correspond  closely  with  the  theoretical  values. 

Neutralization  of  the  Solution. — If  the  solution  should  contain 
free  alkali,  it  may  be  prepared  for  this  test  by  the  following  pre- 
liminary operations: 

(t)  A  test  is  made  for  total  cyanide  by  taking  a  measured 
portion  of  the  liquid,  making  strongly  alkaline,  adding  KI,  and 
titrating  with  Ag]ST03  till  a  distinct  yellow  turbidity  is  produced. 

(ii)  Another  portion  of  the  original  liquid  is  taken,  and  slightly 
more  than  the  amount  of  silver  nitrate  shown  to  be  necessary  by 
the  previous  test  is  added,  together  with  a  slight  excess  of 
potassium  ferrocyanide.  A  few  drops  of  phenol  phthalein  are  added 


26 


CHEMISTRY   OF   CYANIDE   SOLUTIONS. 


to  the  turbid  liquid,  and  the  mixture  titrated  (without  filtering) 
with  N/10  acid,  until  the  pink  tint  just  disappears. 

(ttt)  A  third  portion  of  the  original  liquid  (say  50  c.c.)  is 
taken,  and  the  amount  of  N/10  acid  shown  to  be  necessary  by  test 
(ii)  is  gradually  added,  with  agitation,  from  a  burette.  Five  c.c.  of 
neutral  1  per  cent  potassium  iodide  are  then  added,  and  the  mixture 
titrated  with  AgN03  until  a  distinct  flocculent  precipitate  (white) 
remains  permanent  after  agitation  and  standing  for  about  half  a 
minute.  No  notice  need  be  taken  of  an  obscure  cloudiness  which 
may  appear  in  the  liquid  at  an  earlier  stage. 

The  following  are  some  results  obtained  with  solutions  prepared 
by  mixing  known  volumes  of  simple  cyanides,  zinc  double  cyanides, 
and  other  substances  occurring  in  working  solutions  (ferrocyanides, 
thiocyanates,  chlorides,  etc.). 


Theoretical  Contents. 

Value  of  free  cyanide  obtained  by  white 
turbidity  in  presence  of  KI  in  neutralized 
solution. 

KCy.  % 

Zinc. 
% 

Total 
cyanide. 

% 

Free 
cyanide. 

t 

.039 

.626 

.470 

.46 
.465 
.461 
1.13 
.472 
.457 
.365 
.278 
.199 

(10  c.c.  taken) 
(15  c.c.  taken) 
(25  c.c.  taken) 
(mean  of  4  tests) 

(mean  of  5  tests) 
(mean  of  5  tests) 

.04 
.045 

1.235 
.611 

1.075 
.431 

.07 
.095 
.05 

.612 
.614 
.4 

.332 
.234 
.2 

(E)     Estimation  of  Free  Cyanide  by  Liebig's  Method  in  Presence 
of  Soluble  Sulphides. 

As  above  noted,  the  presence  of  sulphides  in  the  liquid  to  be 
tested  entirely  vitiates  the  result,  owing  to  the  immediate  formation 
of  silver  sulphide. 

(t)  Correction  by  Means  of  Lead  Salts. — In  March,  1893,  Messrs. 
J.  T.  McArthur  and  C.  J.  Ellis  patented  a  process  for  removing 
alkaline  sulphides  from  cyanide  solutions  by  the  use  of  various 
metallic  salts,  recommending  especially  salts  or  compounds  of  lead, 
such  as  plumbates,  carbonates,  acetates  and  sulphates.  They  also 
instanced  the  sulphate  and  chloride  of  manganese,  zincates,  oxide 
and  chloride  of  mercury,  and  ferric  hydrate  or  oxide.  They  also 
applied  lead  salts  for  this  purpose  in  the  testing  of  solutions. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  27 

Carbonate  of  lead  is  probably  the  most  convenient  substance  to 
use,  as  it  introduces  no  soluble  salts  which  might  interfere  with  the 
subsequent  titration.  The  solution  to  be  tested  is  agitated  with 
small  quantities  of  carbonate  of  lead  (best  freshly  precipitated) 
until  further  addition  gives  no  black  or  brown  coloration  to  the 
liquid.  After  settling,  the  clear  solution  (or  an  aliquot  portion  of 
it)  is  filtered  off  and  settled  in  the  ordinary  way.  The  reaction  is 
as  follows  : 

PbC03  +  K2S  =  PbS  +  K2C03. 

Litharge  may  be  employed  instead  of  carbonate  of  lead  with  al- 
most equally  good  results. 

If  a  soluble  salt  of  lead  be  used,  it  will  be  necessary  to  take  a 
measured  volume  of  the  original  liquid,  add  a  slight  excess  of  the 
lead  salt,  make  up  to  a  definite  volume,  allow  to  settle,  and  filter 
off  an  aliquot  part.  If  lead  acetate  be  employed  it  is  necessary  to 
remove  any  excess  by  the  addition  of  an  alkaline  carbonate  before 
finally  making  up  to  a  definite  volume,  otherwise  a  portion  of 
cyanide  might  be  precipitated  as  PbCy2. 

In  case  the  finishing  point  should  not  be  clearly  defined,  owing 
to  a  faint  turbidity  appearing  before  the  true  finish,  it  is  advisable 
to  add  a  little  ammonia  and  potassium  iodide  before  titrating  with 
silver  nitrate. 

(U)  Correction  by  Means  of  Iodine. — W.  J.  Sharwood  (Journal 
Amer.  Chem.  Soc.,  1897,  p.  400)  describes  the  following  process: 

"A  promising  method  for  the  removal  of  sulphide  was  based  on 
the  fact  that  a  weak  solution  of  iodine  precipitates  sulphur  from 
alkaline  sulphides,  this  redissolving  in  a  minute  or  two  to  form 
thiocyanate,  as  the  direct  decomposition  of  potassium  cyanide  by 
iodine  under  these  conditions  approximates  to  the  reaction: 

21  -f  KCN  =  KI  +  ICN 
and  with  K2S  and  KCN  together: 

(1)  21  +  K2S  =  2KI  +  S 

(2)  S  +  KCN  =  KCNS 

so  that  two  atoms  of  iodine  in  either  case  decompose  one  molecule 
of  cyanide,  and  it  seemed  probable  that  the  interference  of  small 
quantities  of  sulphide  could  be  corrected  by  adding  a  constant 
amount  (a  slight  excess)  of  a  weak  iodine  solution  and  introducing 
a  correction  for  the  amount  of  added  iodine,  which  correction  should 
be  independent  of  the  amount  of  sulphide  oxidized.  A  number  of 
preliminary  experiments  confirmed  this,  subsequent  titration  with 


28  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

AgN08  giving,  with  the  correction,  quite  accurate  determinations 
of  the  cyanide  taken,  when  the  amount  of  sulphide  was  small  and 
other  reducing  agents  were  absent;*  with  larger  proportions  of 
sulphide  an  appreciable  error  was  introduced,  the  extent  and  varia- 
tion of  which  were  not  determined." 

In  cases  where  the  amount  of  sulphide  is  large  or  where  other 
reducing  agents  are  also  present,  Sharwood  recommends  the  follow- 
ing :  "Take  twice  the  usual  volume  of  solution,  add  some  soda,  then 
sodium  plumbite  in  very  slight  excess,  shake  well,  make  up  to  a 
definite  volume,  filter,  and  use  half  the  clear  filtrate  for  titration, 
rejecting  the  first  few  c.c.  filtered.  In  presence  of  zinc,  add  a  con- 
siderable excess  of  caustic  soda  or  potash." 


METHOD  No.  2. 

Estimation  of  Free  Cyanide  by  Means  of  a  Solution  of  Iodine  in 
Iodide  of  Potassium. 

This  process  depends  upon  the  fact  that  when  a  solution  of  iodine 
in  potassium  iodide  is  added  to  a  solution  of  a  simple  cyanide,  the 
reddish-brown  color  of  the  iodine  solution  disappears  so  long  as 
the  cyanide  is  in  excess,  since  the  reaction  results  in  the  formation 
of  an  iodide  of  an  alkali  metal  and  cyanogen  iodide,  both  of  which 
are  colorless: 

KCy  +  I2  =  KI  +  ICy. 

The  finish  of  the  reaction  is  sharply  marked  by  the  permanence 
of  the  yellowish  tint  in  the  solution  under  examination,  after  agi- 
tation. 

The  end-reaction  may  be  made  even  more  distinct  by  using  a  drop 
of  the  starch  indicator  prepared  as  follows: 

One  part  of  pure,  fresh  starch  is  rubbed  into  a  thin  paste  with 
a  little  cold  water,  then  gradually  poured  into  150  to  200  times  its 
weight  of  boiling  water,  the  boiling  continued  for  a  few  minutes, 
and  the  liquid  then  allowed  to  stand  and  settle.  Only  the  clear 
solution  is  used  for  the  indicator.  The  finishing  point  is  now 
marked  by  the  permanence  of  an  intense  blue  or  bluish-violet  tint. 
The  starch  solution  does  not  retain  its  sensitiveness  for  long,  and 
must  generally  be  freshly  prepared.  The  substances  sometimes 

*  It  would  apparently  be  simpler.  In  such  cases,  to  determine  the  cyanide  by 
the  'Iodine  method'  of  Fordos  and  Geils,  rather  than  to  complete  the  titration 
by  Liebig's  method. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  29 

added  to  preserve  it,  e.g.,  caustic  alkalis,  zinc  chloride  and  mercuric 
iodide,  would  be  inadmissible  for  testing  cyanide  solutions. 

An  iodine  solution,  suitable  for  cyanide  titrations,  may  be  pre- 
pared by  dissolving  3.903  grams  of  chemically  pure  iodine  and 
about  6  grams  of  potassium  iodide  in  a  small  volume  of  water,  and 
diluting  with  distilled  water  to  1,000  c.c.  With  this  solution,  1  c.c. 
iodine  =  0.01  gram  KCy. 

It  is  not  necessary,  in  practice,  to  prepare  chemically  pure  iodine, 
as  the  solution  may  be  very  conveniently  standardized  by  means  of 
a  solution  of  pure  potassium  cyanide,  the  strength  of  which  has  been 
accurately  ascertained  by  Liebig's  (silver  nitrate)  method.  The 
value  of  the  iodine  solution  is  thus  made  to  depend  on  the  accuracy 
of  the  standard  silver  solution.  For  cyanide  testing,  this  method 
of  standardizing  is  more  convenient  than  the  usual  method  with 
standard  thiosulphate,  and  probably  quite  as  accurate,  if  not  more 
so,  since  nitrate  of  silver  is  easily  obtained  in  a  condition  of  great 
purity. 

The  method  of  titration  with  iodine  is  described  by  Fordos  and 
Gelis  (Journal  de  Chim.  et  de  Pharm.,  23,  48),  and  generally 
ascribed  to  them,  but  the  reaction  on  which  it  depends  appears  to 
have  been  originally  mentioned  by  Terullas  and  Wohler  (Fresenius, 
Quant.  Anal.,  7th  ed.,  1876,  Vol.  I.,  p.  375). 

Limitations  of  the  Method. — This  process  is,  of  course,  not  ap- 
plicable where  other  substances  are  present  which  are  also  capable 
of  reacting  on  iodine,  but  in  some  cases  such  interfering  substances 
may  be  removed,  as  described  below. 

The  iodine  method  is  useful  in  certain  cases  where  the  solution 
is  free  from  zinc,  but  turbid  with  suspended  matter  which  cannot 
conveniently  be  removed  by  filtration.  With  such  a  solution,  titra- 
tion with  silver  nitrate  would  be  difficult,  if  not  impossible,  as  the 
end-point  could  not  be  accurately  observed.  When  the  starch  indi- 
cator is  used,  the  end-point  with  iodine  is  quite  distinct,  even  in 
turbid  solutions. 

In  presence  of  the  zinc  double  cyanide  the  finishing  point  is  un- 
certain and  indefinite.  A  white  precipitate  occurs  gradually  at  a 
certain  stage  of  the  titration,  probably  consisting  of  zinc  cyanide, 
as  follows: 

K2ZnCy4  +  2I2  =  ZnCy2  +  2KI  +  2ICy. 

The  substances  commonly  occurring  in  cyanide  solutions  which 
interfere  with  the  iodine  process  are  caustic  alkalis,  monocarbonates, 


30  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

ammonia,  sulphides,  thiosulphates,  and  probably  most  organic  re- 
ducing agents.  Ferrocyanides,  ferricyanides  and  thiocyanates  (sul- 
phocyanides)  do  not  interfere.  According  to  W.  J.  Sharwood, 
small  quantities  of  sulphides  do  not  interfere,  since  the  sulphur 
liberated  combines  with  an  equivalent  of  cyanide.  (See  above.) 
The  procedure  in  presence  of  alkalis  is  described  below. 

Estimation  of  Free  Cyanide  by  Iodine  in  Presence  of  Alkalis. 

In  the  presence  of  alkalis  the  finishing  point  with  iodine  becomes 
very  indefinite,  and,  moreover,  represents  more  than  the  amount  of 
free  cyanide  present. 

It  was  originally  recommended  to  add  carbonic  acid  water  (ordi- 
nary soda  water),  in  order  to  convert  both  hydrates  and  mono- 
carbonates  into  bicarbonates,  for  example: 
KOH  +  C02  =  KHC03. 
K2C03  +  C02  +  H20  =  2KHC08. 

Since  bicarbonates  have  no  action  on  iodine,  the  solution  may 
now  be  titrated  by  standard  iodine,  and  the  amount  of  cyanide  cor- 
rectly determined. 

Bettel  (Proceedings  Chem.  and  Met.  Soc.  South  Africa,  Vol.  I., 
p.  219)  recommends  adding  50  c.c.  of  ordinary  soda  water  to  the 
solution  to  be  tested,  and  titrating  at  once  with  decinormal  or  centi- 
normal  iodine. 

It  is  evident,  however,  that  any  excess  of  carbonic  acid  will  de- 
compose cyanide  with  liberation  of  hydrocyanic  acid,  thus : 

KCy  +  C02  +  H20  =  KHC03  +  HCy, 

though  probably  with  dilute  solutions,  if  titrated  at  once,  the  effect 
of  a  small  excess  of  C02  would  not  be  very  noticeable. 

However,  the  following  method  (Proceedings  Chem.  and  Met.  Soc. 
South  Africa,  Vol.  I.,  p.  205)  gives  accurate  results,  and  avoids 
the  danger  of  loss  from  the  use  of  excess  of  acid : 

To  a  measured  volume  of  the  solution  silver  nitrate  is  first  added 
until  a  permanent  turbidity  is  produced,  the  exact  amount  added 
being  of  no  consequence.  A  drop  of  phenol  phthalem  is  now  added 
to  this  somewhat  turbid  solution,  and  standard  acid  (for  example, 
N/10  H2S04,  HC1  or  HN03)  is  added  until  the  pink  color  just 
disappears.  The  amount  required  represents,  as  will  be  shown  later, 
the  quantity  of  acid  necessary  to  convert  hydrates  into  neutral  salts, 
and  monocarbonates  into  bicarbonates,  without  any  decomposition 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  31 

of  cyanide  taking  place.  If,  now,  we  take  a  fresh  measured  volume 
of  the  original  solution  and  add  to  it,  with  agitation,  the  quantity 
of  dilute  acid  shown  to  be  necessary  by  the  previous  experiment,  we 
obtain  a  solution  which  may  at  once  be  titrated  with  iodine. 

METHOD  No.  3. 

Estimation  of  Free  Cyanide  by  Means  of  a  Solution  of  Mercuric 
Chloride. 

J.  B.  Hannay  (Journal  Chem.  Soc.,  1878,  Vol.  I.,  p.  245)  de- 
scribes this  method  for  examination  of  commercial  cyanides.  It 
depends  upon  the  fact  that  when  mercuric  chloride  is  added  to  a 
solution  of  free  cyanide  containing  ammonia,  the  precipitate  first 
formed  is  redissolved  as  long  as  the  cyanide  is  in  excess,  the  reactions 
being  probably  as  follows: 

(a)  Precipitation  of  white  mercur-ammonium  chloride : 
HgCl2  +  2NH8  =  NH4C1  +  NH2HgCl. 

(6)  This  precipitate  dissolves,  on  agitation,  as  long  as  cyanide  is 
in  excess. 

NH2HgCl  +NH4C1  +  2KCy  =  HgCy2  =  2KC1  +  2NH3. 

(c)  When  no  more  free  cyanide  remains,  further  addition  of 
HgCl2  gives  a  permanent  precipitate  according  to  reaction  (a). 

Since,  from  these  reactions,  one  molecule  of  HgCl2  is  equivalent  to 
two  of  KCy,  27.074  parts  of  mercuric  chloride  are  required  for 
13.02  parts  of  potassium  cyanide.  Hence  if  a  solution  be  prepared 
containing  20.794  grams  mercuric  chloride  per  liter,  then  1  c.c. 
mercuric  solution  =  0.01  gram  of  KCy.  The  ordinary  N/10  solu- 
tion of  mercuric  chloride  contains  13.537  grams  per  liter.  Hence 
1  c.c.  of  this  =  0.00651  gram  KCy. 

The  method  of  titrating  is  as  follows  (Sutton,  Volumetric 
Analysis,  8th  ed.,  p.  217)  : 

"The  vessel  containing  the  solution  to  be  tested  is  placed  upon 
black  paper  or  velvet ;  ammonia  is  then  added  in  moderate  quantity 
and  the  mercuric  solution  cautiously  added,  with  constant  stirring, 
until  a  bluish-white  opalescence  is  permanently  produced." 

Hannay  states  that  alkaline  sulphates,  nitrates  and  carbonates, 
caustic  alkalis,  cyanates  and  thiocyanates  have  no  effect  on  the  esti- 
mation of  cyanide  by  this  method.  He  further  states  that  if  silver 
nitrate  be  added,  and  then  ammonia  to  dissolve  the  precipitate,  if 
any,  the  cyanogen  may  still  be  completely  estimated  by  titration  with 


32  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

mercuric  chloride.  Hence  any  cyanogen  present  in  the  solution  as 
KAgCy2  would  be  determined  as  free  cyanide,  a  result  which  would 
render  the  method  useless  for  many  practical  purposes. 

The  finishing  point  is  much  less  definite  than  in  the  silver  nitrate 
or  iodine  methods.  I  find  that  a  solution  of  1  per  cent  potassium 
iodide  and  4  per  cent  caustic  soda  makes  a  much  better  indicator 
than  ammonia,  1  c.c.  of  this  indicator  being  added  for  every  10  c.c. 
of  cyanide  solution  taken  for  the  test,  the  end-point  being  marked  by 
the  permanence  of  a  slight  yellowish  tinge. 

METHOD  No.  4. 

Estimation  of  Free  Cyanide  by  Means  of  an  Ammoniacal  Solution 
of  a  Copper  Salt. 

This  is  merely  a  reversal  of  the  well-known  method  for  the  esti- 
mation of  copper  by  means  of  standard  potassium  cyanide.  A  solu- 
tion is  prepared  by  adding  ammonia  to  a  pure  solution  of  sulphate  or 
nitrate  of  copper  until  a  clear,  deep  blue  liquid  is  obtained.  This  is 
standardized  by  running  into  a  measured  volume  of  pure  cyanide 
solution  of  known  strength  until  a  faint  purplish  tint  remains  per- 
manent. An  equal  volume  of  the  solution  to  be  tested  is  then  im- 
mediately titrated  in  the  same  way.  The  strength  of  the  standard 
solution  must  be  checked  frequently  against  pure  cyanide.  I  have 
found  it  generally  preferable  to  add  the  copper  solution  in  slight 
excess  and  determine  the  excess  by  means  of  the  standard  cyanide 
solution. 

This  method  is,  of  course,  subject  to  the  indefiniteness  as  to  the 
exact  finishing  point  which  characterizes  the  determination  of  cop- 
per by  this  means.  The  presence  of  zinc  interferes.  The  reading 
obtained  indicates  much  more  than  the  free  cyanide,  but  does  not 
give  the  whole  of  the  cyanogen  present  as  K2ZnCy4. 

METHOD  No.  5. 
Estimation  of  Free  Cyanide  by  Titration  with  Standard  Acid. 

Potassium  cyanide  is  alkaline  to  most  indicators,  such  as  litmus" 
phenol  phthalein  and  methyl  orange.  Hence  the  amount  of  cyanide 
in  a  pure  solution  may  be  estimated  by  simple  titration  with  an  acid 
until  the  completion  of  the  reaction,  e.g., 

2KCy  +  H2S04  =  K2S04  +  2HCy. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  33 

In  the  case  of  litmus  and  phenol  phthalein,  the  finishing  point 
is  not  quite  sharp,  as  the  indicators  are  affected  to  some  extent  by 
the  hydrocyanic  acid  liberated  during  the  titration.  With  methyl 
orange  tolerably  accurate  determinations  may  be  made.  When 
alkaline  hydrates  and  carbonates  are  present  these  must  be  separately 
determined  and  a  correction  made.  This  matter  will  be  fully  dis- 
cussed in  treating  of  the  estimation  of  alkalis  in  cyanide  solutions. 
Bicarbonates  are  alkaline  to  methyl  orange,  but  neutral  to  the  other 
indicators.  It  may  here  also  be  remarked  that  the  double  cyanide  of 
zinc  and  potassium  is  completely  alkaline  to  methyl  orange,  and 
partially  to  phenol  phthalei'n. 

The  method  cannot  be  recommended  for  practical  use  owing  to 
the  numerous  corrections  necessary,  but  Goyder  (Chemical  News, 
Vol.  LXXIL,  p.  81)  has  attempted  to  apply  it  as  follows: 

"The  titration  is  made  by  measuring  100  c.c.  of  sump  solution,  or 
solution  after  passing  through  the  tailings  into  a  stoppered  bottle, 
adding  1  c.c  of  1/20  per  cent   phenol  phthalein,  and  running  in 
N/10  hydrochloric  acid  till  the  pink  color  is  destroyed." 
1  c.c.  N/10  acid  =  0.0065  gram  KCy. 

John  Longmaid  (Engineering  and  Mining  Journal,  May  11, 
1895,  p.  435)  describes  an  experiment  which  shows  that  under  cer- 
tain conditions  a  solution  which  has  passed  through  a  charge  of  ore 
may  become  perfectly  neutral  to  phenol  phthalein  while  still  appear- 
ing to  contain  large  amounts  of  free  cyanide.  The  experiment, 
however,  is  inconclusive,  as  nothing  is  said  as  to  the  nature  of  the 
foreign  salts  present  in  the  solution  after  use,  and  this  solution  may 
easily  have  contained  considerable  quantities  of  hydrocyanic  acid. 
The  writer's  contention  that  the  alkalinity  of  cyanide  solutions 
towards  phenol  phthalein  is  due  to  a  trace  of  caustic  potash  is  un- 
tenable, since  the  percentage  of  cyanogen  may  be  quantitatively  de- 
termined in  pure  solutions  by  means  of  N/10  acid  and  phenol 
phthalein. 


SECTION  2. 
ESTIMATION  OF  TOTAL  CYANIDE. 

For  the  purpose  of  this  investigation,  the  term  'total  cyanide' 
will  be  used  to  indicate  the  equivalent  in  potassium  cyanide  of  all 
the  cyanogen  existing  in  the  form  of  simple  cyanides,  hydrocyanic 


34  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

acid  and  the  double  cyanides  of  zinc.  In  one  of  the  methods  to 
be  described,  the  cyanogen  present  in  certain  other  easily-decomposed 
double  cyanides  would  likewise  be  included,  e.g.,  those  of  silver  and 
mercury.  Methods  in  which  the  cyanogen  present  as  ferrocyanides, 
ferricyanides  and  sulphocyanides  (thiocyanates)  is  also  included 
will  be  discussed  later  under  'Estimation  of  Total  Cyanogen.' 
The  chief  methods  proposed  are: 

1.  Titration  with  silver  nitrate  after  addition  of  an  excess  of 
caustic  alkali. 

2.  Titration  with  iodine  in  presence  of  an  excess  of  ferrocyanide. 

3.  Titration  with  silver  nitrate  after  decomposition  of  double  cya- 
nides by  means  of  alkaline  sulphide. 

4.  Gravimetric  determination  by  precipitating  with  excess  of  sil- 
ver nitrate  in  presence  of  nitric  acid. 

METHOD  No.  1. 

Estimation  of  Total  Cyanide  by  Titration  with  Silver  after  Addi- 
tion of  Excess  of  Caustic  Alkali. 

When  a  sufficient  quantity  of  caustic  alkali  has  been  added  to  a 
solution  containing  zinc  double  cyanide,  simple  cyanides  of  the  alkali 
and  alkaline  earth  metals,  or  hydrocyanic  acid,  the  whole  of  the 
cyanogen  existing  in  all  or  any  of  these  forms  may  be  determined 
by  titration  with  silver  nitrate.  To  obtain  accurate  results  it  is  neces- 
sary to  use  the  potassium  iodide  indicator.  The  amount  of  caustic 
alkali  to  be  added  will,  of  course,  depend  on  the  amount  of  zinc 
present,  but  an  excess  is  of  no  consequence.  Usually  about  10  c.c. 
of  normal  NaOH  will  be  sufficient.  For  regular  use  it  is  convenient 
to  employ  an  indicator  consisting  of  4  grams  NaOH  and  1  gram  KI 
dissolved  to  100  c.c.  in  distilled  water.  From  5  to  10  c.c.  of  this 
solution  are  added  for  each  test.  The  finishing  point  is  generally 
quite  sharp  and  definite,  and  is  shown  by  a  permanent  yellowish 
tint.  In  some  cases,  as  pointed  out  by  L.  M.  Green  (Inst.  Min.  and 
Met.t  October  17,  1901),  a  white  cloudiness  occurs  before  the  true 
finishing  point,  probably  due  to  precipitation  of  zinc  ferrocyanides. 
I  find  that  in  most  cases  this  may  be  entirely  prevented  by  the  addi- 
tion of  a  little  ammonia.  So  far  as  I  have  been  able  to  ascertain, 
this  method,  which  is  undoubtedly  the  best  for  determining  total 
cyanide  in  practice,  was  suggested  by  the  observation  of  W.  K.  Feldt- 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  35 

mann  (Engineering  and  Mining  Journal,  Vol.  LVIIL,  1894,  pp. 
218-219)  that  "addition  of  alkali  to  working  solutions  which  have 
become  somewhat  weak  in  alkali  brings  up  the  strength  by  regener- 
ating (i.e.,  decomposing)  the  zinc  cyanide,  so  that,  as  a  matter  of 
fact,  when  the  solutions  are  pretty  strongly  alkaline  they  contain  no 
zinc  as  cyanide,  but  only  as  hydrate  dissolved  in  alkali  (zincate  of 
potash,  etc.)."  This  explanation  is  probably  erroneous,  as  will  be 
shown  below,  but  it  is  very  probable  that  it  suggested  the  methods 
described  almost  simultaneously  by  G.  A.  Goyder  (Chemical  News, 
Vol.  LXXIL,  p.  80)  and  W.  Bettel  (Proceedings  Chem.  and  Met. 
Soc.  South  Africa,  Vol.  I.,  p.  165),  in  both  of  which  the  addition 
of  potassium  iodide  is  recommended. 

Goyder  proceeds  as  follows:  "Ten  c.c.  of  5  per  cent  NaOH  is 
mixed  with  20  c.c.  of  the  sump  solution.  If  a  precipitate  is  formed, 
15  c.c.  of  the  mixture  is  filtered  off  and  titrated,  after  addi- 
tion of  potassium  iodide,  with  standard  silver  solution"  (15  c.c. 
solution).  He  says,  further,  that  the  percentage  thus  obtained  indi- 
cates cyanogen  present  as  free  cyanide,  zinc  double  cyanide,  and 
some  other  double  cyanides,  but  does  not  include  ferrocyanides  or 
the  double  cyanides  of  mercury  or  copper. 

Bettel  adds  caustic  alkali  in  excess  (a  few  c.c.  of  normal  caustic 
soda)  to  50  c.c.  of  the  solution  to  be  tested,  and  a  few  drops  of  a 
10  per  cent  solution  of  potassic  iodide,  and  titrates  to  opalescence 
with  AgN03.  This  gives  free  cyanide,  hydrocyanic  acid,  and  double 
cyanides  such  as  zinc  potassium  cyanide,  but  not  ferrocyanides  or 
sulphocyanides. 

W.  J.  Sharwood  (Journal  Amer.  Chem.  Soc.,  1897,  pp.  400-434) 
discusses  this  method  of  titration  in  detail,  and  says:  "If  zinc  be 
present,  a  large  excess  of  alkali  should  be  added;  in  this  case  the 
cyanogen  found  represents  not  only  the  potassium  cyanide,  but  also 
the  double  zinc  compound.  By  estimating  the  zinc,  the  amount  of 
free  potassium  cyanide  may  be  readily  calculated  as  1  part  of  zinc 
corresponds  with  4  parts  of  potassium  cyanide.  A  similar  allowance 
must  be  made  if  small  quantities  of  copper  are  present.  If  calcium, 
magnesium  or  manganese  are  present,  ammonium  chloride  must  be 
added,  whilst  soda  is  used  in  presence  of  aluminium  or  lead."  He 
also  recommends  the  addition  of  ammonia  (Engineering  and  Mining 
Journal,  1898,  p.  216) :  "Total  cyanogen  was  obtained  by  continu- 
ing the  titration  with  silver  after  addition  of  caustic  soda  and  a  little 


36  CHEMISTEY    OF    CYANIDE    SOLUTIONS. 

ammonia  and  potassium  iodide;  this,  however,  does  not  include 
cyanogen  in  double  cyanides  of  copper,  silver,  gold  and  mercury." 

Nature  of  the  Reaction  in  Presence  of  Zinc. — The  precise  nature 
of  the  reaction  taking  place  when  caustic  alkalis  are  added  to  solu- 
tions containing  zinc  double  cyanide  has  given  rise  to  much  discus- 
sion. According  to  Feldtmann's  observations,  quoted  below,  it 
should,  apparently,  be : 

K2ZnCy4  +  4KOH  =  Zn(OK)2  +  4KCy  +  2H20. 

As,  however,  the  reverse  reaction  undoubtedly  takes  place,  it  seems 
uncertain  whether  free  potassium  cyanide  and  potassium  zincate 
can  coexist  in  the  same  solution. 

Goyder  (Chemical  News,  Vol.  LXXIL,  p.  80),  commenting  on 
Feldtmann's  observation,  remarks :  "But  I  believe  that  caustic  alkali 
is  never  added  in  large  excess  to  the  lixiviating  solutions,  and  when 
added  in  small  quantities  the  double  decomposition  would  not  be 
complete,  and  its  amount  could  only  be  calculated  by  applying  the 
laws  of  chemical  mass-action  after  finding  the  relative  proportion  of 
the  double  cyanide  of  zinc  and  potassium  to  caustic  alkali,  and  the 
velocity  of  combination  of  the  resulting  salts.  As,  in  practice,  this 
problem  is  complicated  by  the  presence  of  the  double  salts  and  caus- 
tic potash,  as  well  as  other  salts,  its  solution  is  probably  impossible. 
It  may,  however,  be  taken  for  granted  that  when  caustic  alkali  is 
added  to  a  solution  of  double  cyanide  of  potassium  and  zinc  in 
molecular  proportions,  the  resulting  solution  will,  after  a  little  time, 
contain  zincate  of  potash,  cyanide  of  potassium  and  the  double  salt." 

Bettel  (Proceedings  Chem.  and  Met.  Soc.,  South  Africa,  Vol.  I., 
p.  167)  also  gives  the  reaction  between  zinc  double  cyanide  and 
caustic  alkali  as  above,  and  also  the  analogous  reaction  for  alkaline 
carbonates : 

K2ZnCy4  +  4Na2C03  -f  2H20  = 

2KCy  +  2NaCy  +  Zn(NaO)2  -f  4NaHC03. 

J.  S.  C.  Wells,  however  (Engineering  and  Mining  Journal,  Vol. 
LX.,  p.  584),  states  definitely  that  K2ZnCy4  is  not  decomposed 
by  alkali  into  free  KCy  and  potassium  zincate. 

Charles  J.  Ellis  (Journal  Soc.  Chem.  Ind.,  January  28,  1897, 
p.  117)  remarks  as  follows: 

"I  have  proved  the  following  reactions  to  take  place  on  adding 
silver  nitrate  to  a  solution  of  double  zinc  cyanide  in  absence  of 
free  alkali: 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  37 

ZnCy2'KCy  -f  AgNO,  =  AgCy'KCy  +  KN03  -f  ZnCy2,  and 

ZnCy2-2KCy  +  2AgN03  =  2AgCy  +  2KN03  +  ZnCy2." 

[I  may  here  mention  that  I  have  on  several  occasions  verified  this 
observation.  The  precipitate  formed  by  adding  to  a  solution  of 
K2ZnCy4  half  the  amount  of  silver  nitrate  required  for  determina- 
tion of  total  cyanide  by  the  process  under  discussion,  when  collected 
and  carefully  washed,  was  found  to  consist  exclusively  of  zinc  cya- 
nide ZnCy2.] 

"We  may  suppose  the  following  reactions  to  take  place  simul- 
taneously in  presence  of  caustic  alkali: 

(a)  2ZnCy2-2KCy  +  2AgN03  —  2AgCyKCy  -f  2KN03+  2ZnCy2. 
(6)  2ZnCy2  +  4KOH  =  ZnCy/2KCy  +  ZnOK20  +  2H20. 
The  potassic  cyanide  portion  of  the  double  zinc  cyanide,  re-formed 
as  in  (&)  being  acted  upon  further  by  the  silver  salt  as  in  (a),  this 
going  on  until  all  the  zinc  is  brought  into  the  form  of  zinc-potassic 
oxide." 

The  observations  of  Wells  and  Ellis  show  that  it  is  not  at  all  neces- 
sary to  assume  any  such  decompositions  as  those  given  by  Feldtmann, 
Goyder  and  Bettel.  The  formation  of  zincate  may  be  considered 
as  taking  place  only  on  addition  of  sufficient  silver  nitrate,  the  entire 
reaction  being  expressible  as  follows : 
K.ZnCy,  +  4KOH  -f  2AgN03  = 

2KAgCy2  +;  Zn(OK)2  +  2KN03  +  2H20. 
Or,  in  the  case  of  carbonates, 

K2ZnCy4  -f  4Na2C03  -f  2AgN03  +  2H20  = 

2KAgCy2  +  Zn(ONa)2  -f  2NaN03  +  4NaHC03. 

Bettel  gives  the  following  equation,  in  absence  of  caustic  or  car- 
bonated alkalis : 

20K.,ZnCy4  +  3AgN"03  = 

3KAgCy2  +  3KN03  +  2[(ZnCy2)10(KCy)17]. 

Disturbing  Factors  in  the  Titration  of  Solutions  Containing  Zinc. 
— G.  A.  Goyder  (Chemical  News,  Vol.  LXXIL,  p.  80)  makes  the 
following  remarks  on  titration  in  presence  of  zinc  cyanide : 

"In  titrating  the  sump  solutions  which  contain  much  of  their 
cyanogen  as  the  double  cyanide  of  zinc  and  potassium,  the  end- 
reaction  was  not  only  ill-defined,  but  the  quantity  of  nitrate  of  silver 
required  to  produce  a  permanent  turbidity  increases  with  dilution, 
with  the  temperature,  and  also  with  the  amount  of  simple  cyanide 
added  to  a  greater  extent  than  was  calculated.  ...  A  cold 
sump  solution  to  which  nitrate  of  silver  has  been  added  to 


38  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

permanent  turbidity  always  clears  on  being  heated.  As  regards  the 
indefiniteness  of  the  reaction,  a  sample  of  sump  solution  was  divided 
into  three  equal  portions.  To  No.  2  an  equal  volume  and  to  No.  3 
two  volumes  of  distilled  water  were  added,  and  these  were  given 
to  an  expert  well  acquainted  with  the  process,  but  not  knowing  how 
the  solutions  were  made  up  to  test.  He  reported  that  No.  1  con- 
tained 0.04  per  cent,  No.  2,  0.05  per  cent  and  No.  3,  0.04  per  cent  of 
simple  cyanide."  A  solution  which,  when  tested,  appeared  to  con- 
tain 0.02  per  cent  was  strengthened  by  the  addition  of  more  cyanide 
until  it  should,  by  calculation,  have  contained  0.09  per  cent.  The 
actual  test,  however,  indicated  a  strength  of  0.15  per  cent. 

The  indication  in  presence  of  zinc,  when  an  excess  of  alkali  is  not 
added,  thus  appears  to  be  a  function,  not  only  of  the  free  cyanide, 
but  also  of  (a)  the  degree  of  dilution;  (&)  the  temperature;  (c)  the 
rate  of  titration;  (d)  the  amount  of  zinc  present;  (e)  the  amount 
of  ferrocyanide  present. 

In  a  given  solution,  provided  it  can  be  obtained  perfectly  clear,' 
it  is  possible  to  distinguish  the  following  indications  with  silver 
nitrate : 

1.  The  first  indication  of  a  flocculent  precipitate,  on  titrating  a 
measured  quantity    (undiluted)     with  AgN03  without  indicator. 
This  can  be  observed,  although  with  some  difficulty,  with  a  pure 
solution  of  K2ZnCy4. 

2.  The  indication  in  presence  of  neutral  potassium  iodide — gen- 
erally slightly  higher  than  No.  1. 

3.  The  indication  in  presence  of  a  slight  excess  of  ferrocyanide. 

4.  The  indication  with  excess  of  caustic  alkali  and  potassium 
iodide. 

5.  The  indication  with  ammonia  and  potassium  iodide. 
To  these  might  be  added  : 

6.  The  indication  with  neutral  potassium  chromate  as  indicator. 
This  will  be  discussed  later. 

Titration  of  Cyanide  in  Solutions  Containing  Copper. — Accord- 
ing to  the  observations  of  Goyder  and  Sharwood  (quoted  above)  any 
cyanogen  existing  as  double  cyanide  of  copper  and  potassium  will 
not  be  determined  at  all  when  the  potassium  iodide  indicator  is  used. 
A  marked  difference  is  observed  in  the  result,  according  as  potassium 
iodide  is  added  or  not.  This  matter  has  been  discussed  in  consider- 
able detail  by  W.  H.  Virgoe  (Proceedings  Inst.  Min.  and  Met.,  Vol., 
X.,  p.  102)  and  myself  (Journal  Soc.  Chem.  Ind.,  Vol.  XIX., 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  39 

p.  14).  Hence  it  appears  that,  when  titrated  without  addition  of 
potassium  iodide,  a  certain  proportion  of  the  cyanogen  of  the 
copper  salt  is  determined  as  though  it  were  free  KCy.  The 
amount  so  determined  is  increased  by  dilution  or  addition  of  alkalis. 
In  solutions  containing  no  free  alkali,  about  3/14  of  the  total  cyanide 
of  the  copper  salt  is  determined  by  titration  with  AgN03  without 
indicator  (assuming  that  no  portion  of  it  is  determined  by  titration 
in  presence  of  KI).  The  end-point  in  presence  of  the  iodide  is 
slightly  affected  by  dilution,  but  is  not  altered  by  the  presence  of 
alkali.  The  average  of  a  large  number  of  experiments  showed  that 
3.5  are  consumed  (or  at  least  rendered  incapable  of  reacting  with 
silver  nitrate  in  presence  of  KI)  for  every  part  of  copper  dissolved, 
when  the  copper  salt  is  prepared  with  precautions  to  prevent  loss  of 
cyanogen. 

A  tentative  explanation  of  these  reactions  may  be  based  on  the 
assumption  that  cuprous  cyanide,  Cu2Cy2,  forms  molecular  com- 
pounds with  KCy.  Thus  we  may  suppose  the  reaction  of  cyanide 
on  cupric  hydrate  to  be  as  follows: 

2Cu(HO)2  +  7KCy  =  5KCyCu2Cy2  -f  2KOH. 

When  silver  nitrate  is  added  to  the  solution  of  this  double  salt, 
a  portion  only  of  the  combined  potassium  cyanide  is  capable  of  re- 
acting to  form  the  soluble  double  cyanide  of  silver: 

4(5KCyCu2Cy2)  +  3AgNO.=  3KAgCy2  -f  3KN03  -f- 
2(7KCy2Cu2Cy2). 

When  potassium  iodide  is  present,  this  reacts  forming  silver  iodide 
before  any  of  the  copper  salt  is  decomposed. 

Addition  of  water  or  alkali  apparently  renders  further  quantities 
of  cyanide  in  the  copper  salt  accessible  to  silver  nitrate  (in  absence 
of  KI). 

It  will  be  noted  that  in  presence  of  copper  and  a  small  excess  of 
alkali,  the  end-point  in  presence  of  KI  is  lower  than  without  it, 
whereas  with  zinc  and  a  small  amount  of  alkali  the  reverse  is  the 
case. 

METHOD  No.  2. 

Estimation  of  Total  Cyanide  by  Means  of  Iodine  in  Presence  of  an 
Excess  of  Ferrocyanide. 

As  already  pointed  out,  the  indications  of  the  iodine  method  of 
Fordos  and  Gelis  are  indistinct  and  unreliable  for  the  determination 


40  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

of  free  cyanide  in  presence  of  zinc.  The  total  cyanide  (i.e.,  the 
equivalent  of  the  cyanogen  as  KCy  and  K2ZnCy4,  calculated  as  KCy) 
may  be  determined,  however,  with  tolerable  accuracy  by  the  follow- 
ing modification  of  this  method : 

After  removing  the  excess  of  alkali  by  addition  of  the  requisite 
quantity  of  dilute  acid  (determined  as  explained  above),  the  solution 
is  mixed  with  a  moderate  excess  of  potassium  ferrocyanide.  On 
titrating  the  resulting  mixture  with  a  standard  solution  of  iodine  in 
potassium  iodide,  the  whole  of  the  free  cyanide,  together  with  the 
equivalent  in  KCy  of  all  the  cyanogen  present  in  combination  with 
zinc,  is  indicated,  the  end-point  being  fairly  sharp  and  definite.  In 
this  reaction  the  zinc  appears  to  be  precipitated  as  ferrocyanide. 
The  following  equation  is  suggested : 

2ZnK2Cy4  +  K4FeCy6  +  8I2  = 
Zn2FeCy6  +  SKI  +  8ICy. 

This  method  was  found  in  some  cases  to  yield  very  accurate  results. 
It  cannot,  of  course,  be  applied  in  presence  of  other  bodies  capable 
of  reacting  on  iodine.  (Chemical  News,  Vol.  LXXIL,  p.  227.) 

METHOD  No.  & 

Estimation  of  Total  Cyanide  after  Precipitation  of  the  Metals  with 
Alkaline  Sulphides. 

This  method  (Chemical  News,  Vol.  LXXL,  p.  274)  depends  on 
the  facts:  (a)  That  certain  metallic  cyanides,  such  as  zinc  double 
cyanide,  K2ZnCy4,  silver  double  cyanide,  KAgCy2,  and  mercuric 
cyanide,  HgCy2,  are  decomposed  by  sulphuretted  hydrogen  or  an 
alkaline  sulphide,  with  precipitation  of  the  metal  as  sulphide. 
(b)  That  the  excess  of  sulphide  may  be  removed,  without  affecting 
the  cyanides  by  the  addition  of  insoluble  compounds  of  lead,  such  as 
the  oxides  and  carbonates,  or  soluble  alkaline  salts,  such  as  the 
plumbates,  double  tartrates,  etc. 

In  the  case  of  zinc,  the  precipitation  is  not  perfect,  owing  to  the 
slight  solubility  of  zinc  sulphide  in  alkaline  cyanides,  but  the  result 
is  sufficiently  close  for  most  practical  purposes. 

With  silver  the  precipitation  is  so  complete  that  the  method  might 
be  used  for  the  quantitative  estimation  of  silver  in  cyanide  solutions, 
cyanide  being  liberated  (as  free  KCy)  in  exact  proportion  to  the 
silver  precipitated. 


CHEMISTEY    OP    CYANIDE    SOLUTIONS.  41 

The  copper  double  cyanides  are  generally  supposed  not  to  be  de- 
composed by  soluble  sulphides,  but  in  dilute  solutions  the  copper  is 
precipitated  to  a  small  extent. 

The  method  is  as  follows: 

The  solution  to  be  tested  is  mixed  with  a  slight  excess  of  a  pure, 
fairly  concentrated  solution  of  sodium  sulphide,  well  shaken,  and 
allowed  to  stand  until  the  precipitate  has  subsided.  A  little  lime 
may  be  added  to  assist  the  settlement,  in  which  case  the  liquid  may 
be  filtered  without  difficulty.  A  definite  volume  is  taken;  e.g.,  if 
the  cyanide  solution  originally  taken  measured  100  c.c.,  it  is  made  up 
by  addition  of  alkali  solution,  sodium  sulphide  and  water  to  200  c.c., 
and  of  this,  100  c.c.  are  removed  by  filtration.  This  portion  is 
agitated  with  litharge  or  carbonate  of  lead,  which  is  best  added  in 
small  quantities  at  a  time,  with  constant  agitation,  until  a  drop  of 
the  filtered  liquid  fulfils  the  following  conditions: 

(a)  Does  not  give  the  slightest  black  or  brown  coloration  with  a 
drop  of  a  solution  of  a  lead  salt.  A  perfectly  white  precipitate  of 
lead  cyanide  should  be  produced.  Lead  tartrate  (dissolved  in  alkali) 
gives  no  precipitate  with  cyanides.  (&)  Gives  no  precipitate  with 
sodium  carbonate,  (c)  Gives  no  precipitate  with  alkaline  sulphides. 
A  brown  coloration  may  be  produced,  probably  owing  to  the  solution 
of  a  small  quantity  of  the  lead  compound  in  the  alkaline  liquid. 

When  these  conditions  are  fulfilled,  a  definite  volume  is  again 
filtered  off,  say,  50  c.c.,  which  in  this  case  would  represent  25  c.c. 
or  one-quarter  of  the  original  cyanide  solution.  This  is  titrated 
with  silver  nitrate  in  the  usual  manner — best  with  addition  of  potas- 
sium iodide.  It  often  happens  that  a  slight  granular  precipitate 
is  observed  towards  the  finish,  and  it  is  necessary  to  add  the  last 
few  drops  of  silver  nitrate  slowly,  with  agitation.  The  end-point 
may  be  made,  however,  perfectly  definite  by  the  addition  of  am- 
monia and  potassium  iodide. 

It  is  to  be  particularly  observed  that  this. method  indicates  the 
cyanogen  present  as  KAgCy2  (which  is  not  shown  by  titration  with 
AgN03  with  alkaline  iodide  indicator),  in  addition  to  free  KCy  and 
KCy  as  K2ZnCy4. 

When  an  excess  of  ferrocyanide  is  present,  the  zinc  is  not  precipi- 
tated, or  only  to  a  slight  extent,  by  addition  of  sulphide ;  if,  however, 
the  liquid  be  made  strongly  alkaline  by  the  addition  of  caustic  soda, 
and  well  shaken,  the  zinc  is  almost  completely  converted  into  sul- 
phide. In  some  other  cases,  also,  the  zinc  fails  to  precipitate  imme- 


42 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


diately,  but  the  precipitation  generally  takes  place  on  agitation, 
especially  if  the  liquid  be  made  alkaline. 

A.  F.  Crosse  (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol. 
III.,  p.  164)  states  that  the  precipitation  of  zinc  from  K2ZnCy4 
by  means  of  alkaline  sulphides  is  incomplete  in  the  cold,  but  prac- 
tically complete  at  65°  C. 

EXPERIMENTS  ON  THE  ESTIMATION  OF  CYANOGEN  IN  COMPOUND 
CYANIDES  AFTER  ADDITION  OF  ALKALINE  SULPHIDE. 

In  the  following  tests,  known  Quantities  of  silver  nitrate  were 
added  to  a  measured  quantity  of  cyanide  solution  of  known  strength, 
then  sodium  sulphide  added  until  the  silver  salt  was  partially  de- 
composed, the  mixture  made  up  to  a  definite  volume,  filtered,  and 
the  cyanide  strength  determined  by  titration  with  silver  nitrate. 

TEST  No.  1. 
Mixture  taken. 

Potassium  cyanide,  1.4%  KCy 50  c.c. 

Silver  nitrate,  1.304%  AgN03 25  c.c. 

Sodium  sulphide,  0.27%  Na2S 5  c.c. 

Water    .                                                         .  20  c.c. 


100  c.c. 


Theoretical  Contents. 


Before  adding 
sodium  sulphide. 

After  adding 
sodium  sulphide. 

0  450  gram 

0  495  gram 

KCy  equivalent  of  KAgCya 

gram 

gram 

0  700  gram 

Results  of  Titration. 


Volume  of 
mixture 

Indicator 
added. 

Standard 
AgNO,  req. 

Free  cyanide  per 
100  parts  of 

taken. 

mixture. 

c.c. 

c.c 

Grams. 

10 

none 

4.00 

0.49  1 

10 

none 

4.95 

0.495  1  mean, 

8 

neutral  KI 
neutral  KI 

4.90 
5.00 

0.49    f  0.494 
0.50  J 

CHEMISTRY    OF    CYANIDE    SOLUTION'S. 

TEST  No.  2. 
Mixture  taken. 

Potassium  cyanide,  1.4%  KCy 50  c.c. 

Silver  nitrate,  1.304%  AgN03 25  c.c. 

Sodium  sulphide,  0.27%  Na2S 15  c.c. 

Water    .                                                        .  10  c.c. 


43 


100  c.c. 


Theoretical  Contents. 


Before 
adding  Na,S 
Grama. 

After 
adding  Na2S 
Grams. 

0450 

0585 

0  250 

0  115 

0700 

0700 

Result  of  Titration. 


Volume  of 
mixture  taken. 

c.c. 

Indicator 
added. 

AgNO3 
standard  sol. 
required, 
c.c. 

Free  cyanide  indi- 
cated per  100  c.c. 
of  mixture. 
Grams. 

10 
10 
10 

None 
KI 
KI 

5.85 
5.80 
5.85 

0.585 
0.580 
0.585 

TEST  No.  3. 
Mixture  taken. 


Potassium  cyanide,  1.4%  KCy. 
Silver  nitrate,  1.304%  AgN03 . 
Sodium  sulphide,  0.27%  Na2S. 


100  c.c. 


44 


CHEMISTRY   OF   CYANIDE   SOLUTIONS. 

Theoretical  Contents. 


Before 
adding  Na,S 

Grams. 

After 
adding  Na,S 

Grams. 

id 

0  450 

0  g~5 

0  250 

0  025 

0  700 

0700 

Result  of  Titration. 


Volume  of 
mixture  taken. 

Indicator 
added. 

Standard  AgNO» 
required. 

Free  cyanide  indi- 
cated per  100  c.c. 
of  mixture. 

c.c. 

C.C. 

Grams. 

10 
10 
10 

None 
KI 
KI 

6.75 
6.80 
6.70 

0.675 
0.680 
0.670 

In  the  three  tests  above,  the  following  reactions  are  assumed : 

(1)  2KCy  +  AgN08  =  KAgCy2  +  KN03. 

(2)  2KAgCy2  +  Na2S  =  Ag2S  +  2KCy  +  2NaCy. 

In  the  following  tests,  to  which  zinc  as  well  as  silver  was  added, 
it  is  assumed  that  the  following  reaction  occurs  after  the  comple- 
tion of  reaction  (2)  when  sufficient  Na2S  has  been  added: 

(3)  K2ZnCy4  +  Na2S  =  ZnS  +  2KCy  +  2NaCy. 

On  titrating  the  resulting  solution  with  AgN03,  after  making 
strongly  alkaline  and  adding  KI,  all  the  cyanogen  existing  as  KCy, 
NaCy  or  K2ZnCy4  should  be  indicated  as  its  equivalent  of  KCy. 

TEST  No.  4. 
Mixture  taken. 

Potassium  cyanide,  \A%  KCy 50  c.c. 

Zinc  sulphate,  \%  Zn 10  c.c. 

Silver  nitrate,  1.304%,  AgN08 15  c.c. 

Sodium  sulphide,  0.33%  Na2S 10  c.c. 

Water  ..  .15  c.c. 


100  c.c. 


CHEMISTRY  OF   CYANIDE   SOLUTIONS. 

Theoretical   Contents. 


Before 
adding  Na,S 

Grams. 

After 
adding  Na,S 

Grams. 

015 

026)  na 

KCY  equivalent  of  K»ZnCy4 

040 

040fa66 

KCy  equivalent  of  KAgCy,  

0  15 

0  04 

Total  cyanide  

070 

070 

Result  of  Titration. 


Cyanide  indicated  per  100  parts  of  mixture. 


Volume  of 
solution  tested. 

Without 
KI 

With  neutral 
KI 

With  alkaline 
KI 

Grams. 

Grams. 

Grams. 

10  c.c.  in  each  case. 

0.37 

0.495 
0  50 

0.655 
0  goo 

0655 

TEST  No.  5. 
Mixture  taken. 

Potassium  cyanide,  1.23%  KCy 50  c.c. 

Zinc  sulphate,  1%  Zn 10  c.c. 

Silver  nitrate,  1.304%,  AgN03 20  c.c. 

Sodium  sulphide,  0.33%  Na2S '. 10  c.c. 

Water  ..  .10  c.c. 


100  c.c. 


Theoretical  Contents. 


Before 
adding  Na,S 

Grams. 

After 
adding  Na,S 

Grams. 

0015 

0  125  I  n 

KCy  equivalent  of  KsZnCy4  

0400 

0400)0-885 

KCy  equivalent  of  KAgCyt           .           . 

0200 

0090 

Total  cyanide  

0615 

0  615 

46 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

Result  of  Titration. 


Cyanide  Indi 

cated  per  100  parts 

of  mixture. 

Volume  of 
mixture  taken, 

Without 
KI 

With  neutral 
KI 

With  alkaline 
KI 

Grams. 

Grams. 

Grams. 

10  c.c.  in  each  case. 

0.27 

0.385 

0.525 
0.525 

TEST  No.  6. 
Mixture  taken. 

Potassium  cyanide,  0.54%  KCy 25  c.c. 

Sodium  zincate,  0.1%  Zn 20  c.c. 

Sodium  sulphide,  0.156%  Na2S 20  c.c. 

Water  .  35  c.c. 


100  c.c. 


Theoretical  Contents. 


Before 
adding  Na2S 

Grams. 

After 
adding  Na»S 

Grams. 

id 

0055 

0  135 

0  080 

Total   cyanide         

0  135 

0  135 

Solution  treated  with  lime  and  filtered,  then  treated  with  lead 
carbonate  and  filtered  again. 

Result  of  Titration. 


Volume 
of  filtrate  taken 
for  test. 

C.C. 

Volume  of 
AgNOi  required  with 
alkaline  KI  as  indicator. 

c.c.* 

Cyanide  indicated 
per  100  parts 
of  mixture. 

Grama 

50 
40 

12.9 
10.4 

0.129 
0.130 

1  c.c.  AgNOt  B0lutlon=0.005  gram  KCy. 


CHEMISTBY    OP    CYANIDE    SOLUTIONS. 

TEST  No.  7. 


47 


A  solution  was  prepared  having  the  following  theoretical  composi- 
tion: 

Free  cyanide,  KCy 0.2% 

KCy  equivalent  of  K2ZnCy4 0.2% 

Total  cyanide  (as  KCy) 0.4% 

Ferrocyanide  (as  K4FeCy9'3H20) 0.2% 

Thiocyanate  (as  KCyS) 0.04% 

Bicarbonate    (as  NaHC08) 0.39% 

Carbonate  (as  Na2C03) 0.06% 

Chloride  (as  NH4C1) 0.1% 

The  zinc  was  added  in  the  form  of  zinc  sulphate  (solution  con- 
taining 0.5  per  cent  Zn) . 

(a)  Test  for  total  cyanide  by  direct  titration  with  silver  nitrate, 
using  alkaline  iodide  indicator,  showed  0.4  per  cent  KCy. 

(6)  Tests  were  made  by  taking  50  c.c.  of  the  above  mixture,  and 
adding  excess  of  sodium  sulphide.  No  precipitate  of  zinc  sulphide 
occurred  until  the  liquid  was  made  strongly  alkaline  by  adding  lime 
and  agitating. 

The  mixture  was  made  up  to  100  c.c.,  filtered,  the  filtrate  agitated 
with  1  gram  of  lead  carbonate,  and  again  filtered,  then  tested  with 
AgN03  and  alkaline  iodide  indicator. 


Volume  of 
original  solu- 
tion taken. 

Volume  of 
filtrate 
tested. 

Fraction 
of  original 
solution. 

Volume  of 
AgXO. 
required. 

Amount 
of  KCy 
Indicated. 

c.c. 

c.c. 

C.C. 

% 

50 
50 

90 
90 

9/10 
9/10 

34.20 
33.95 

0.380 
0.377 

SECTION  3. 
ESTIMATION  OF  TOTAL  CYANOGEN. 

The  term  total  cyanogen  will  be  used  to  indicate  all  the  cyan- 
ogen present  in  the  solution,  whether  in  the  form  of  simple  cyanides, 
double  cyanides,  hydrocyanic  acid  or  compounds  such  as  ferro-  and 


48  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

ferricyanides,  sulphocyanides,  cyanates  and  isocyanates.     Some  of 
the  methods  here  described  are  not  actually  universal,  but  the  sub- 
stances excepted  (of  which  the  cyanogen  cannot  be  determined  by 
the  process  in  question)  are  of  unusual  occurrence  in  practice. 
The  methods  detailed  below  are : 

1.  The  volumetric  method  of  Vielhaber,  by  titrating  with  silver 
nitrate  and  potassium  chromate  indicator. 

2.  Gravimetric  method  with  silver  nitrate  and  ammonia. 

3.  Gravimetric  method  with  oxide  of  mercury. 

4.  The  total  cyanogen  may  also  be  determined  by  estimating  the 
carbon  and  nitrogen  obtained  in  the  ultimate  analysis  of  the  com- 
pounds by  the  methods  of  organic  analysis,  for  which  reference 
must  be  made  to  works  on  that  subject. 

METHOD  No.  1. 

Estimation  of  Total  Cyanogen  by  Titration  with  Silver  Nitrate, 
Using  the  Chromate  Indicator. 

This  process,  described  by  Vielhaber  (Arch.  Pharm.  [3],  XIII., 
408),  is  merely  an  adaptation  of  the  ordinary  volumetric  method 
of  estimating  chlorides  devised  by  Mohr.  Chlorides  are,  of  course, 
estimated  along  with  the  cyanogen  compounds,  and  when  present 
must  be  separately  determined. 

The  method  is  as  follows : 

Two  or  three  drops  of  a  saturated  solution  of  neutral  (yellow) 
potassium  chromate,  K2Cr04,  are  added  to  the  solution  to  be  titrated, 
and  the  standard  silver  solution  run  in  until  a  faint  reddish  tinge 
appears  and  remains  permanent  on  shaking.  With  a  pure  solution 
of  a  simple  cyanide,  twice  as  much  silver  nitrate  solution  must  be 
added,  as  in  Liebig's  method,  in  order  to  obtain  the  reaction,  since 
the  indication  with  chromate  does  not  occur  until  the  whole  of  the 
cyanide  has  been  precipitated,  as  follows : 

(a)  2KCy  +  AgN03  =:  KAgCy2  +  KN03, 

(6)  KAgCy2  +  AgN03  =  2AgCy  +  KNO, ; 
after  which  (in  the  absence  of  chlorides  and  other  cyanogen  com- 
pounds) red  silver  chromate  is  precipitated  as  follows: 

(c)  K2Cr04  +  2AgNO,  =  Ag2Cr04  +  2KN08. 

The  solution  must  not  contain  any  considerable  amount  of  free 
alkali.  If  this  be  present,  it  must  be  first  neutralized  by  the  addition 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  49 

of  the  requisite  amount  of  dilute  sulphuric  or  nitric  acid  (not 
hydrochloric  acid),  which  may  be  determined  as  already  described. 

When  hydrocyanic  acid  is  present  Vielhaber  recommends  neutral- 
izing with  magnesium  carbonate  suspended  in  water. 

The  reactions  with  commonly-occurring  substances  precipitable 
by  silver  nitrate  in  a  cyanide  solution,  occur  more  or  less  in  the 
following  order,  all  these  reactions  being  completed  before  the  chrom- 
ate  color  becomes  permanent : 

(1)  2KCy  +  AgN03  =  KAgCy2  +  KN03. 

(2)  KAgCy2  +  AgN03  =  2AgCy  +  KN03. 

(3)  KC1  -f-  AgN03  =  AgCl  +  KN03. 

(4)  KSCy  +  AgN03  =  AgSCy  +  KN03. 

(5)  K4FeCya  +  4AgN03  =  Ag4FeCy6  +  4KN03. 
K2ZnCy4  +  AgN03  =  ZnCy2  +  KAgCy2  +  KNO,. 
ZnCy2  +  2AgN03  =  Zn(N03)2  +  2AgCy. 

The  reactions  taking  place  with  zinc  double  cyanides  on  the  addi- 
tion of  silver  nitrate  in  presence  of  an  excess  of  ferrocyanide  are 
discussed  by  L.  M.  Green  in  a  paper  laid  before  the  Institute  of 
Mining  and  Metallurgy,  October  17,  1901  (Proceedings,  Vol.  X.,  p. 
29). 

Isocyanates,  if  present,  will  also  be  precipitated  by  silver  nitrate 
before  the  chromate  reaction  is  obtained.  Cyanates  and  bicarbon- 
ates  are  apparently  not  precipitated.  Carbonates  obscure  the  finish- 
ing point  somewhat  (acting  probably  in  the  same  way  as  caustic 
alkalis),  but  the  chromate  reaction  occurs  before  the  carbonate  has 
been  precipitated  by  silver  as  Ag2C03. 

Chloride  of  silver  may  be  separated  from  the  cyanide  by  boiling 
with  concentrated  nitric  acid,  which  decomposes  the  cyanide  but 
leaves  the  chloride  undissolved. 

METHOD  No.  2. 

Gravimetric  Determination  of  Total  Cyanogen  by  Means  of  Silver 
Nitrate  and  Nitric  Acid. 

Fresenius  (Quant.  Anal.,  7th  ed.,  Vol.  I.,  p.  375)  describes  the 
following  method,  due  to  H.  Hose : 

"Digest  for  some  time  with  a  dilute  solution  of  nitrate  of  silver, 
stirring  frequently,  then  add  nitric  acid  in  moderate  excess,  and 
digest  at  a  gentle  heat,  till  the  foreign  cyanide  is  fully  dissolved 


50  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

and  the  cyanide  of  silver  has  become  pure  and  white.  (Double 
cyanide  of  nickel  and  potassium  yields  by  this  process  a  mixture  of 
cyanide  of  silver  with  cyanide  of  nickel.  Like  double  cyanides  are 
similarly  decomposed.)  Then  add  water  and  filter.  As  a  pre- 
cautionary measure  it  is  well  to  test  the  metal  obtained  by  long 
ignition  of  the  cyanide  of  silver,  whether  it  is  free  from  those 
metals  which  were  combined  with  the  cyanogen.  The  filtrate  is  used 
for  estimating  the  bases,  the  silver  being  first  precipitated  with 
hydrochloric  acid.  This  method  affords  an  exact  analysis  of  the 
double  cyanides  of  potassium  with  nickel,  copper  and  zinc." 

I  may  here  remark  that  I  have  used  a  practically  identical 
method,  with  good  results,  for  the  estimation  of  copper  in  cyanide 
solutions,  the  copper  being  determined  in  the  filtrate  by  a  colori- 
metric  method. 

The  precipitated  cyanide  of  silver  may  be  collected  on  a  weighed 
filter,  dried  at  100°  and  weighed,  or  it  may  be  collected  on  an  un- 
weighed  filter  and  converted  into  metallic  silver  by  igniting  in  a 
porcelain  crucible  for  a  quarter  of  an  hour,  or  until  it  ceases  to  lose 
weight. 

The  following  remarks  (Fresenius,  Quant.  Anal.,  7th  ed.,  Vol., 
I.,  p.  141)  on  the  properties  of  cyanide  of  silver  may  be  of  use  in  this 
connection:  "Cyanide  of  silver  may  be  dried  at  100°  without  de- 
composition; it  is  soluble  in  ammonia.  Exposure  to  light  fails  to 
impart  the  slightest  tinge  of  black  to  it.  Upon  ignition  it  is  decom- 
posed into  cyanogen,  which  escapes,  and  metallic  silver,  which  re- 
mains mixed  with  a  little  paracyanide  of  silver.  By  boiling  with  a 
mixture  of  equal  parts  of  sulphuric  acid  and  water,  it  is,  according 
to  Glassford  and  Napier,  dissolved  to  sulphate  of  silver,  with  libera- 
tion of  hydrocyanic  acid." 

It  is  to  be  observed  that  this  method  of  precipitation  with  silver 
nitrate  in  presence  of  excess  of  nitric  acid  cannot  conveniently  be 
employed  in  presence  of  ferrocyanides,  as  they  yield  a  precipitate 
of  silver  ferrocyanide  and  not  of  AgCy,  and  on  ignition  this  leaves 
some  oxide  of  iron,  together  with  metallic  silver.  Chlorides  also  in- 
terfere; chloride  of  silver  is  formed,  which  does  not  decompose  on 
ignition  for  one-quarter  hour  at  a  moderate  temperature. 

When  thiocyanates  are  present  in  the  original  liquid  the  precipi- 
tate will  contain  silver  thiocyanate,  which  on  ignition  will  yield 
metallic  silver  proportional  to  the  cyanogen. 

From  these  considerations  it  will  be  seen  that  in  dealing  with 


CHEMISTBY    OF    CYANIDE    SOLUTIONS.  51 

impure  solutions,  the  method  is  more  useful  as  a  means  of  eliminat- 
ing cyanogen  as  a  preliminary  step  for  other  determinations  than 
as  a  means  of  determining  cyanogen  itself. 

METHOD  No.  3. 

Gravimetric  Determination  of  Total  Cyanogen  by  Means  of  Silver 
Nitrate  and  Ammonia. 

W.  Weith  (Zeitschrift  f.  analyt.  CJiem.,  9,  379)  recommends 
a  solution  of  nitrate  of  silver  in  ammonia  for  the  decomposition  of 
many  cyanogen  compounds,  such  as  ferrocyanide  of  potassium, 
Prussian  blue,  and  even  cobalticyanide  of  potassium.  He  digests 
them  in  sealed  tubes  at  100°  C.  (in  the  case  of  cobalticyanide  of 
potassium  at  150°  C.)  for  4  or  5  hours.  The  contents  of  the 
tube  are  then  gently  warmed  in  a  dish  until  the  crystals  of 
ammonia-cyanide  of  silver  are  dissolved.  Any  separated  metallic 
oxide  is  filtered  off  and  washed  with  ammonia.  The  filtrate  is 
diluted,  and  cyanide  of  silver  precipitated  therefrom  by  acidifying 
with  nitric  acid.  This  may  be  collected  and  determined  as  in  the 
previous  method.  The  method  would  probably  be  chiefly  applicable 
to  the  solid  cyanogen  compounds,  rather  than  to  solutions. 

In  the  filtrate  the  silver  may  be  separated  from  the  alkalis,  etc. 
In  respect  to  the  undissolved  oxides,  it  should  be  noted  that  metallic 
silver  is  always  mixed  with  the  oxide  of  iron. 

METHOD  No.  4. 

Gravimetric  Determination  of  Total  Cyanogen  by  Boiling  with 
Oxide  of  Mercury. 

The  following  account  of  this  method  is  given  by  Fresenius 
(Quant.  Anal,  7th  ed.,  Vol.  I.,  p.  376) : 

"Many  simple  cyanides,  and  also  double  cyanides,  both  of  the 
character  of  the  double  cyanide  of  nickel  and  potassium  and  of  the 
ferro-  and  ferricyanide  (not,  however,  cobalticyanides)  may,  as  is 
well  known,  be  completely  decomposed  by  boiling  with  excess  of 
oxide  of  mercury  and  water,  all  cyanogen  being  determined  as 
cyanide  of  mercury  and  the  metals  passing  into  oxides." 

H.  Hose  has  shown  that  Prussian  blue,  ferro-  and  ferricyanide  of 


52  CHEMISTBY    OF    CYANIDE    SOLUTIONS. 

potassium,  more  particularly,  may  be  readily  analyzed  in  this  man- 
ner: 

"Boil  a  few  minutes  with  water  and  excess  of  oxide  of  mercury 
until  complete  decomposition  is  effected,  and,  in  order  to  render 
the  sesquioxide  of  iron  and  oxide  of  mercury  removable  by  the 
filter,  nitric  acid  in  small  portions,  till  the  alkaline  reaction  has 
nearly  disappeared,  filter,  wash  with  hot  water,  dry  the  precipitate, 
ignite,  very  gradually  raising  the  heat  under  a  hood  with  a  good 
draught,  and  weigh  the  sesquioxide  of  iron  remaining." 

The  cyanogen  is  determined  in  the  filtrate  as  follows : 

"Mix  the  solution  (containing  the  cyanogen  as  cyanide  of  mer- 
cury) with  nitrate  of  zinc  dissolved  in  ammonia.  To  one  part  of 
mercury  salt  add  about  two  parts  of  the  zinc  salt.  Add  to  the 
clear  solution  sulphuretted  hydrogen  water  gradually  till  it  pro- 
duces a  perfectly  white  precipitate  of  sulphide  of  zinc.  The  precip- 
itate, which  is  a  mixture  of  the  sulphides  of  mercury  and  zinc, 
settles  well.  After  a  quarter  of  an  hour  filter  it  off  and  wash  with 
very  dilute  ammonia.  The  filtrate  contains  cyanide  of  zinc  dis- 
solved in  ammonia,  together  with  nitrate  of  ammonia.  It  does  not 
smell  of  hydrocyanic  acid,  and  consequently  no  escape  of  the  latter 
takes  place.  Mix  it  with  nitrate  of  silver  and  then  add  dilute  sul- 
phuric acid  in  excess.  The  cyanide  of  silver  is  next  washed  a  little 
by  decantation,  then — to  free  it  from  any  cyanide  of  zinc  simul- 
taneously precipitated — heated  with  a  solution  of  nitrate  of  silver, 
finally  filtered  off,  washed  and  weighed,  after  drying  at  100°,  as 
AgCy  or  (after  ignition)  as  metallic  silver." 

The  cyanogen  in  the  filtrate  from  the  precipitate  of  mixed  sul- 
phides could,  of  course,  be  determined  volumetrically  by  titration 
with  silver  nitrate  and  potassium  iodide  indicator. 


SECTION  4. 
ESTIMATION  OF  HYDROCYANIC  ACID. 

Free  hydrocyanic  acid  may  be  estimated  by  titration  with  silver 
nitrate 

(a)  After  addition  of  an  excess  of  caustic  alkali. 

(6)  After  addition  of  an  excess  of  bicarbonate. 

Detection  of  hydrocyanic  acid : 

The  presence  of  any  considerable  quantity  of  hydrocyanic  acid  in 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  53 

a  solution  may  be  detected  by  its  smell,  or  by  covering  a  beaker  con- 
taining a  little  of  the  solution  with  a  watch-glass,  on  the  under 
surface  of  which  is  a  drop  of  silver  nitrate.  If  the  drop  rapidly 
becomes  milky,  the  presence  of  hydrocyanic  acid  is  indicated. 

Solutions  containing  the  free  acid  are  best  measured  by  means  of 
a  graduated  column.  If  a  pipette  be  used  a  plug  of  cotton  wool 
slightly  moistened  with  silver  nitrate  should  be  inserted  into  the 
upper  end,  to  avoid  the  danger  of  inhaling  the  acid. 

METHOD  No.  I. 

Estimation  of  Hydrocyanic  Acid  by  Silver  Nitrate  with  Addition 
of  Alkali. 

This  modification  of  Liebig's  method,  suggested  originally  by 
Siebold,  is  described  by  Sutton  (Volum.  Anal,  8th  ed.,  p.  217). 
The  liquid  to  be  titrated  is  measured  from  a  burette  dipping  into  a 
solution  of  caustic  alkali  contained  in  a  beaker.  The  resulting 
alkaline  cyanide  is  then  titrated  with  silver  nitrate  in  the  ordinary 
way.  A  large  excess  of  alkali  must  be  avoided,  unless  the  potas- 
sium iodide  indicator  be  used. 

If  the  original  solution  also  contains  free  cyanide,  i.e.,  cyanides 
of  the  alkali  or  alkaline  earth  metals,  a  separate  determination  must 
be  made  by  titrating  with  silver  nitrate  without  adding  caustic 
alkali;  the  result  of  this  titration,  deducted  from  that  of  the  first, 
gives  the  equivalent  in  terms  of  KCy  of  the  hydrocyanic  acid  present. 
It  may  here  be  remarked  that  the  finishing  point  in  presence  of  free 
hydrocyanic  acid  is  somewhat  indefinite. 

This  method  is,  of  course,  not  applicable  in  presence  of  easily  de- 
composed double  cyanides,  such  as  K2ZnCy4,  as  these  would  be  de- 
termined along  with  the  cyanide  resulting  from  the  HCy,  after 
''•'  addition  of  alkali.  Ferrocyanides  and  sulphocyanides  should  not  in- 
terfere. 

METHOD  No.  2. 

Estimation  of  Hydrocyanic  Acid  by  Silver  Nitrate  after  Adding 
an  Alkaline  Carbonate. 

This  method,  described  by  Bettel  (Proceedings  Chem.,  Met.  Soc. 
South  Africa,  Vol.  I.,  p.  165),  is  said  to  be  applicable  in  presence 
of  double  cyanides  such  as  K2ZnCy4. 


54  CHEMISTBY    OP    CYANIDE    SOLUTIONS. 

The  free  cyanide  is  first  estimated  in  the  ordinary  way,  without 
addition  of  alkali. 

Another  portion  of  the  liquid  is  then  taken,  and  a  solution  of 
potassium  or  sodium  bicarbonate  free  from  monocarbonate  is 
added.  Free  carbonic  acid  must  be  absent.  The  resulting  liquid  is 
then  titrated  with  silver,  and  the  result  of  the  first  titration  de- 
ducted from  the  number  found.  This  gives  the  equivalent  in  KCy 
of  the  hydrocyanic  acid  present. 

The  method  depends  on  the  reaction : 

2KHC03  +  AgN03  +  2HCy  =  KAgCy2  +  KN03  +  2C02  +  2H20, 
and  is  therefore  a  reversal  of  Siebold's  method  of  estimating  car- 
bonates by  means  of  hydrocyanic  acid  and  silver  nitrate.  (Year 
Book  of  Pharmacy,  1878,  p.  518.)  Bicarbonates  have  no  action  on 
potassium  or  sodic  zinc  cyanide. 

Instead  of  silver  nitrate,  an  iodine  solution  could  presumably 
be  used  where  interfering  substances  are  absent,  the  reaction  being 
KHC08  +  Ia  +  HCy  =  KI  +  ICy  +  C03  +  H20. 


SECTION  5. 
ESTIMATION  OF  AVAILABLE  CYANIDE. 

The  efficiency  of  a  cyanide  solution  for  dissolving  gold  and  silver 
depends  on  various  other  factors  besides  the  percentage  of  cyanogen 
in  the  form  of  simple  cyanides.  The  amount  of  precious  metal 
dissolved  in  any  particular  case  will  be  determined  by  the  following 
considerations:  (a)  Time  of  contact.  (6)  Quantity  of  solution 
in  proportion  to  metal  to  be  dissolved,  (c)  Temperature,  (d) 
Physical  condition  of  the  metal  acted  upon,  (e)  Nature  of  sur- 
rounding bodies ;  for  example,  the  presence  or  absence  of  other  sub- 
stances which  might  be  attacked  by  cyanide.  (/)  Amount  and 
nature  of  other  salts  in  solution,  some  of  which  may  either  assist 
or  hinder  the  action  of  cyanide,  or  may  be  themselves  solvents  or 
precipitants  of  the  precious  metals,  (g)  Amount  of  dissolved 
oxygen  in  the  solution,  (h)  Size  or  shape  of  the  containing  vessel 
— this  being  chiefly  of  importance  in  view  of  the  admission  or  ex- 
clusion of  air.  (t)  Mode  of  application  of  the  solution,  e.g.,  by 
percolation,  agitation,  etc. 


CHEMISTEY    OF    CYANIDE    SOLUTIONS.  55 

It  has  been  shown  that  the  double  cyanide  of  zinc  and  potassium 
is  to  some  extent  a  solvent  of  gold,  though  much  less  efficient  than  a 
simple  cyanide,  for  equal  quantities  of  cyanogen. 

Ferro-  and  ferricyanides  may  also  act  directly  in  some  instances. 
Hydrocyanic  acid  is  also  a  possible  solvent.  In  some  of  these  cases, 
particularly  that  of  K2ZnCy4,  it  is  not  very  clear  whether  the  action 
may  not  be  due  (as  suggested  by  T.  K.  Rose,  Metallurgy  of  Gold, 
p.  398)  to  dissociation  of  the  compound  with  liberation  of  free 
cyanide.  Careful  experiments  (Proceedings  Inst.  Min.  &  Met.,  Vol. 
VI.,  p.  120)  appear  to  have  proved  that  cyanogen  is  not  a  solvent 
of  gold. 

The  researches  of  Maclaurin  (Journal  Chem.  Soc.,  Vol.  LXVII.) 
and  others  have  demonstrated  that  the  efficiency  of  pure  solutions 
depends  on  the  amount  of  dissolved  oxygen  as  well  as  upon  the  per- 
centage of  cyanide,  and,  in  fact,  that  when  solutions  of  various 
strengths  are  compared  under  otherwise  similar  conditions,  there  is 
a  certain  strength  which  shows  a  maximum  efficiency.  Thus  a 
solution  of  0.25  per  cent  was  found  to  dissolve  gold  and  silver  more 
rapidly  than  any  other,  either  stronger  or  weaker.  This  result  ap- 
pears to  be  due  to  the  fact  that  the  solubility  of  oxygen  in  cyanide 
solutions  diminishes  as  the  percentage  of  cyanide  is  increased,  so 
that  the  conditions  most  favorable  for  the  occurrence  of  the  reac- 
tions 

4KCy  -f  2Au  +  0  +  H20  ==  2KAuCy2  +  2KOH, 
4KCy  +  2Ag  -f  0  -f  H20  =  2KAuCy8  -f  2KOH, 
is  obtained  with  this  particular  degree  of  concentration.    Between 
the  limits  0.1  per  cent  and  0.25  per  cent  the  efficiency  was  found  to 
be  nearly  constant.    It  is  obvious,  therefore,  that  a  mere  determina- 
tion of  free  cyanide  gives  us  no  information  as  to  the  efficiency  of 
a  solution  unless  other  circumstances  are  taken  into  consideration, 
for  a  1  per  cent  solution  might  have  actually  less  efficiency  than  a 
0.1  per  cent  solution. 

In  many  cases  some  method  of  rapidly  and  accurately  estimating 
the  solvent  power  of  a  solution  (under  given  conditions)  would  be 
of  the  utmost  value.  Unfortunately  no  thoroughly  reliable  method 
exists,  and  we  are  practically  obliged  to  fall  back  upon  comparative 
extraction  tests  which  really  amount  to  an  experimental  treatment 
of  the  ore  or  material  under  examination,  in  which  actual  working 
conditions  are  imitated  as  closely  as  possible. 

Several  methods  have,  however,  been  tried  by  different  experi- 


56  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

menters,  depending  on  the  rate  of  solution  of  small  quantities  of 
metal  in  the  liquid  to  be  examined,  and  some  description  of  these 
will  be  given  below. 

The  term  'available  cyanide*  (or  perhaps  better  'solvent  activity') 
might  be  defined  as  follows : 

"Two  solutions  are  said  to  have  the  same  solvent  activity  when 
equal  volumes  dissolve  the  same  amounts  of  precious  metal  in  the 
same  time  and  under  the  same  conditions."  If  we  take  a  certain 
solution  for  a  standard  (say,  for  example,  a  0.25  per  cent  solution 
of  pure  potassium  cyanide  in  well  aerated  distilled  water) ,  we  might 
represent  the  weight  of  gold  dissolved  by  the  standard  solution  as 
100,  and  that  dissolved  by  some  other  solution  might  then  be  ex- 
pressed in  terms  of  a  percentage  of  this  standard.  Thus  if  Wj  and 
w0  be  the  weights  dissolved  by  a  given  solution  and  the  standard  re- 
spectively, then  the  solvent  activity  of  the  given  solution  would  be 
expressed  by  100  X  (wx  -f-  w0). 

It  is  obvious  that  the  term  can  only  be  used  in  reference  to  a 
particular  set  of  conditions,  and  that  comparisons  can  only  be 
made  where  all  essential  points  are  the  same  in  all  tests. 

Descriptions  of  experimental  treatment  tests  hardly  come  within 
the  scope  of  a  paper  on  the  'analysis  of  cyanide  solutions' ;  for  these 
reference  may  be  made  to  various  works  on  the  cyanide  process. 

Method  for  the  Estimation  of  Available  Cyanide  by  the  Kate  of 
Solution  of  Metallic  Gold  or  Silver. 

This  method,  described  by  Bettel  (Proceedings  Chem.  and  Met. 
Soc.,  South  Africa,  Vol.  I.,  p.  165),  for  equal  intervals  of  time,  re- 
spectively, in  the  solution  to  be  tested  and  in  standard  solutions  of 
different  strengths,  and  determining  the  loss  of  weight  in  each  case, 
gave  varied  and  discordant  results  ( J.  S.  Maclaurin,  Journal  Chem. 
Soc.,  Vol.  LXVIL,  p.  199),  as  it  is  practically  impossible  to  secure 
uniform  conditions.  Moreover,  such  results  could  not  be  safely  ap- 
plied in  estimating  the  relative  efficiency  of  solutions  for  another 
purpose  (as,  for  example,  for  the  treatment  of  some  particular  ore). 

I  have  recently  experimented  on  a  method  of  comparing  the 
solvent  activity  of  solutions  capable  of  giving  results  which  may 
be  of  some  value,  with  comparatively  little  trouble,  and  in  a  reason- 
ably short  time.  Six  or  more  tests  might  be  executed  simultane- 
ously in  about  an  hour. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  57 

A  solution  of  chloride  of  gold  is  prepared  by  dissolving  pure 
metallic  gold  in  aqua  regia,  evaporating  to  dryness  on  a  water  bath, 
and  dissolving  in  pure  water  to  a  definite  volume.  A  solution  con- 
taining 0.5  milligram  Au  per  c.c.  is  a  convenient  strength.  A 
number  of  exactly  similar  flasks  are  got  ready,  and  an  equal  volume 
(say  10  c.c.)  of  the  gold  chloride  solution  added  to  each.  A  few 
drops  of  a  concentrated  solution  of  sulphur  dioxide  in  distilled 
water  is  added  to  each  flask,  and  the  liquid  then  boiled  till  there  is 
no  smell  of  S02.  The  whole  of  the  gold  will  be  thus  precipitated 
in  a  fine  powder,  and  the  conditions  in  each  flask  will  be  as  nearly 
as  possible  identical.  Add  to  each  a  few  c.c.  of  a  N/10  solution  of 
sodium  hydrate,  so  that  the  liquid  in  the  flasks  becomes  faintly 
alkaline.  Cool,  and  then  add  to  the  flasks  equal  volumes  of  the 
various  solutions  to  be  compared  (at  least  two  tests  should  be  made 
with  distilled  water) .  The  flasks  are  then  agitated  at  intervals  for  15 
minutes,  and  the  residual  gold  collected  on  filters,  and  washed  once 
or  twice  with  distilled  water  until  the  washings  are  free  from  cya- 
nide. The  filter  papers  are  dried  in  porcelain  dishes  on  a  sand  bath 
(or  in  any  other  convenient  way),  wrapped  in  lead  foil  and  cupelled. 
The  resulting  gold  beads  are  weighed,  and  the  solvent  activity  ob- 
tained by  calculation.  The  amount  of  gold  dissolved  is  equal  to  the 
weight  of  bead  from  test  with  distilled  water,  less  the  weight  of  bead 
from  test  with  given  solution.  This  method  introduces  no  foreign 
bodies  except  small  quantities  of  alkaline  chlorides  and  sulphates 
and  of  sodium  hydrate,  which  are  quite  inactive,  and  are,  moreover^ 
practically  present  in  equal  amount  in  all  the  tests. 


58  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

CLASS  II. 
ALKALINE  CONSTITUENTS. 

General  Remarks  on  the  Action  of  Substances  found  in  Cyanide 
Solutions  Toward  Different  Indicators. 

Before  proceeding  to  discuss  the  methods  of  estimating  the 
alkalinity  of  working  cyanide  solutions,  it  is  necessary  to  point  out 
the  behavior  of  the  various  bodies  likely  to  be  found  in  such  solu- 
tions towards  different  indicators. 

The  estimation  of  alkalinity  is  generally  made  by  means  of  a 
dilute  standard  solution  of  some  mineral  acid  (H2S04,  HC1  or 
HN08).  Sometimes  an  organic  acid  (for  example,  oxalic  acid) 
may  be  used.  The  indicators  most  commonly  employed  are  litmus, 
phenol  phthalein  and  methyl  orange,  and  these  alone  will  be  con- 
sidered in  the  following  remarks.  The  methods  of  preparing  these 
indicators  and  their  general  characteristics  are  fully  described  by 
Sutton  (Volum.  Anal,  8th  ed.,  p.  33). 

Methyl  orange  should  not  be  used  when  organic  acids  are  em- 
ployed for  the  titration.  A  convenient  strength  is  0.1  per  cent,  in 
aqueous  solution,  a  single  drop  being  generally  sufficient  for  each 
test.  The  tint  is  then  very  pale  yellow  in  alkaline  solutions  and 
pink  in  acid. 

Phenol  phthalein  is  used  in  the  form  of  an  alcoholic  solution  (in 
methylated  spirit  or  60  per  cent  alcohol).  About  0.5  per  cent  is  a 
convenient  strength.  The  tint  is  deep  pink  (rose  color)  in  alkaline 
solution  and  colorless  in  acid. 

1.  Action  Toward  Simple  Cyanides. — Free  potassium  cyanide 
and  other  cyanides  of  the  alkali  metals  behave,  when  neutralized  by 
dilute  mineral  acids  (and  some  organic  acids),  as  if  the  whole  of 
the  alkali  metal  existed  in  the  form  of  hydrate.  When  methyl 
orange  is  used  as  the  indicator  the  end-point  is  fairly  sharp;  with 
litmus  and  phenol  phthalein  it  is  somewhat  indefinite,  as  these  indi- 
cators are  affected  by  the  hydrocyanic  acid  evolved,  but  with  care 
the  amount  of  standard  acid  required  will  be  found  to  correspond 
pretty  closely,  whatever  indicator  be  used,  to  the  completion  of  the 
reaction : 

KCy  -f  HR'  =  HCy  -f  KR', 
or  its  equivalent,  R  being  any  monovalent  acid  radicle. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  59 

2.  Action  Toward  Alkaline  Hydrates. — Alkaline  hydrates  (e.g., 
caustic  potash,  KOH,  caustic  soda,  NaOH,  lime,  Ca(OH)2,  etc.) 
are  alkaline  to  all  indicators,  the  amount  of  standard  acid  used  cor- 
responding to  the  total  amount  of  the  alkali  metal  present  and  indi- 
cating the  completion  of  the  reaction : 

M'OH  +  HRT  =  H20  -f  M'R',  where 
M  =  any  positive  monovalent  radicle  or  its  equivalent,  and 
R  =  any  negative  monovalent  radicle  or  its  equivalent. 

3.  Action    Toward    Alkaline    Monocarbonates.  —  These     (e.g., 
Na2C03,  K2C03)  are  also  alkaline    to    the    mineral    acids  (HC1, 
HN03,  H2S04),  and  some  organic  acids,  but  the  amount  of  acid 
required  to  effect  the  change  in  the  indicator  depends  on  the  nature 
of  the  indicator  used. 

With  methyl  orange  the  whole  of  the  alkali  metal  is  indicated 
as  though  it  were  present  as  hydrate,  the  permanent  pink  tint  only 
appearing  after  the  completion  of  the  reaction : 

K2C08  +  2HR  =  H20  +  C02  +  2KR, 
or  its  equivalent. 

With  litmus  the  reaction  in  the  cold  is  always  incomplete,  owing 
to  the  action  of  the  liberated  carbonic  acid  on  the  indicator.  If 
the  titration  be  performed  with  the  solution  to  be  examined  at 
boiling  temperature,  the  whole  of  the  alkali  metal  of  the  carbonate 
may  be  estimated. 

With  phenol  phthale'in  the  finishing  point  is  also  somewhat  in- 
definite owing  to  the  same  cause,  but  it  is  possible  to  estimate  by 
means  of  this  indicator,  with  tolerable  accuracy,  the  amount  of 
monocarbonate  in  dilute  solutions,  the  finishing  point  being  the 
same  as  though  half  the  alkali  metal  of  the  carbonate  existed  in  the 
form  of  hydrate,  thus  corresponding  with  the  completion  of  the 
reaction : 

K2C03  •+  HR  =  KHC03  +  KR, 
or  its  equivalent. 

Ammonium  carbonate  behaves  in  a  similar  manner,  but  the  end 
point  with  phenol  phthale'in  is  uncertain,  owing  to  free  ammonia 
present  in  the  solution. 

4.  Action  Toward  Bicarbonates. — The  bicarbonates  of  the  alkali 
and  alkaline  earth  metals  (NaHC03,  KHC03,  CaH2(C03)2)  are 
neutral  to  phenol  phthale'in  and  to  litmus,  but  alkaline  to  methyl 
orange,  the  whole  of  the  alkali  metal  being  indicated  as  though 
present  as  hydrate  according  to  the  equation: 


60  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

KHC03  H-  HR  =  H20  +  C02  -f  KR, 
or  its  equivalent. 

5.  Action  Toward  Ammonia. — Ammonia  is  alkaline  to  all  indi- 
cators, but  cannot  be  accurately  titrated  with  phenol  phthalein. 

6.  Action  Toward  Zinc  Double  Cyanides. — The  double  cyanide 
of  zinc  and  potassium  behaves,  when  titrated  with  standard  acid, 
using  methyl  orange  as  indicator,  as  though  the  whole  of  the  cyan- 
ogen existed  as  free  potassium  cyanide,  the  finishing  point  indi- 
cating the  completion  of  the  reaction: 

K2ZnCy4  +  4HR  =  ZnR2  -f-  2KR  +  4HCy. 
[With  phenol  phthalein  the  alkalinity,  according  to  Bettel  ( Jour- 
nal Chem.  and  Met.  Soc.  South  Africa,  Vol.  I.,  pp.  163-165), 
corresponds  to  19.5  parts  of  potassium  cyanide  out  of  a  possible 
130.2  parts,  i.e.,  to  7.9  per  cent   of  the  total  amount  of  K2ZnCy4 
present     The  following  reaction  is  provisionally  suggested : 
lOKjZnCy,  +  3HC1  =  3HCy  +  3KC1  +  (ZnCy2)10.(KCy)17]. 

7.  Action     Toward     Alkaline     Zincates.  —  Potassium    zincate, 
Zn(OK)2,  behaves  to  methyl    orange    and    phenol    phthalein  as 
though  the  alkali  metal  existed  entirely  as  hydrate. 

8.  Action  Toward  Alkaline  Sulphides. — The  sulphides  of  the 
alkali  metals  (K2S,  Na2S,  presumably,  also,  CaS,  BaS  and  similar 
bodies)  react  towards  acids  with  methyl  orange  indicator  as  though 
the  alkali  metal  existed  as  hydrate : 

K2S  -f  2HR  =  2KR  +  HaS. 

With  phenol  phthalein  in  a  cold  solution,  the  neutral  point  in- 
dicates half  the  alkali  metal  present: 

K2S  +  HR  =•  KHS  -f  KR. 

With  boiling  solutions  the  reaction  is  the  same  as  with  methyl 
orange. 

The  effects  are  thus  precisely  analogous  to  those  obtained  with 
monocarbonates. 

9.  The  following  substances  are  neutral  to  all  indicators : 
Double  cyanides  of  silver  (KAgCy^  NaAgCy2). 

Ferrocyanides, 


Ferricyanides, 

Sulphocyanides, 

Chlorides,* 

Sulphates, 

Nitrates, 


when  present  as  normal  salts  of  the  alkali 
or  alkaline  earth  metals. 


•  Including  ammonium  chloride. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  61 

From  the  foregoing  remarks  it  will  be  seen  that,  of  the  three  in- 
dicators, methyl  orange  is  the  most  sensitive  to  alkalis,  and  phenol 
phthalem  the  most  sensitive  to  acids.  By  a  combination  of  tests 
with  these  two  indicators  we  may  sometimes  determine  several  dif- 
ferent ingredients  in  the  solution. 

Definition  of  'Total'  and  'Protective'  Alkali. — The  'total  alkali' 
of  a  solution  will  be  denned  as  the  equivalent,  in  terms  of  caustic 
potash  (KOH),  of  all  the  ingredients  which  are  alkaline  to  methyl 
orange.  It  is,  therefore,  the  sum  of  the  alkalinities  due  to  the 
several  substances  which  are  alkaline  to  this  indicator.  The  chief 
of  these  are: 
Simple  cyanides, 
Hydrates, 


Carbonates, 
Bicarbonates, 


of  the  alkali  and   alkaline  earth   metals 
Na,  K,  NH4,  Ca,  Ba,  etc. 


Sulphides, 

Zincates, 

Double  cyanides  of  zinc, 

Free  ammonia, 

The  term  'protective  alkali'  is  based  on  the  assumption  that  cer- 
tain ingredients  in  an  ordinary  working  solution  will  be  wholly  or 
partially  neutralized,  on  addition  of  a  dilute  mineral  acid,  or  car- 
bonic acid,  before  any  decomposition  of  cyanide  occurs.  In  the 
treatment  of  ores  the  addition  of  lime  or  caustic  soda  is  made  with 
the  object  of  protecting  the  cyanide  from  the  decomposing  effect  of 
various  matters  contained  in  the  ore,  and  of  the  carbonic  acid  in  the 
air.  As  a  matter  of  fact,  a  cyanide  solution  undergoes  gradual 
decomposition,  with  evolution  of  hydrocyanic  acid,  even  when  an 
excess  of  caustic  alkali  is  present,  so  that  the  protection  afforded  is 
only  partial  and  temporary,  and  not  absolute.  There  is,  however, 
a  fairly  definite  point  in  the  neutralization  of  an  ordinary  solution 
(containing  free  cyanide,  hydrates,  carbonates,  etc.)  at  which  the 
decomposition,  with  formation  of  hydrocyanic  acid,  begins  to  take 
place  with  marked  rapidity.  In  the  absence  of  zinc  this  point 
corresponds  with  tolerable  exactness  to  the  alkalinity  of  the  hydrates 
and  carbonates  present  toward  phenol  phthalei'n,  and  if  the  amount 
of  acid  corresponding  to  this  alkalinity  be  added,  with  agitation,  to 
such  a  solution,  the  cyanide  strength,  as  shown  by  immediate  titra- 
tion  with  silver  nitrate,  will  remain  apparently  unchanged.  In 
this  case,  therefore,  protective  alkali  =  hydrate  -f-  £  carbonate. 


62  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

When  zinc  is  present  the  effect  of  adding  dilute  acid  is  somewhat 
remarkable,  and  has  been  referred  to  in  detail  in  the  discussion  on 
Mr.  L.  M.  Green's  paper  on  the  Titration  of  Cyanide  Solutions  Con- 
taining Zinc.'  No  loss  of  cyanogen  (and  hence  no  evolution  of 
HCy)  takes  place  even  on  prolonged  exposure,  unless  the  acid  added 
exceeds  a  certain  amount. 

The  various  determinations  of  alkaline  constituents  here  consid- 
ered will  be  as  follows : 

(A)  Total  Alkali. — Titration  by  standard  acid  and  methyl  orange 
indicator. 

(B)  Protective  Alkali. — (1)  Titration  with  standard  acid  and 
phenol  phthalein  after  adding  silver  nitrate. 

2.  Titration  with  the  same,  after  adding  excess  of  potassium 
ferrocyanide. 

3.  Distillation  with  potassium  bisulphate. 

(C)  Hydrates. — (1)  By  combined  titrations  with  standard  acid 
and  the  two  indicators,  making  the  necessary  corrections  for  cyanides 
and  carbonates. 

2.  By  titration  with  standard  acid  after  precipitation  of  the 
cyanide  with  AgN03  and  the  carbonate  with  BaCl2. 

(D)  Carbonates  and  Bicarbonates. — (1)  By  calculation  from  re- 
sults of  the  preceding  tests. 

2.  By  precipitation  as  BaC03  and  estimation  of  the  C02  corre- 
sponding to  precipitate  found. 

(E)  Ammonia   (and  Ammonium  Salts). — By  colorimetric  test 
with  Nessler's  solution  after  precipitation  of  cyanogen  compounds 
and  distillation  of  the  nitrate. 

(A)  ESTIMATION  OF  TOTAL  ALKALI. 

A  measured  volume  (say  50  c.c.)  of  the  solution  is  placed  in  a 
flask  with  a  few  drops  of  a  0.1  per  cent  solution  of  methyl  orange. 
N/10  acid  (HC1,  HN03  or  H2S04)  is  run  in  from  a  burette  until  a 
faint  permanent  pink  tint  is  produced. 

When  the  solution  contains  double  cyanides  of  zinc,  copper,  silver, 
etc.,  a  white  precipitate  occurs  at  a  certain  stage  on  the  addition  of 
acid,  consisting  of  the  simple  cyanides  of  these  metals.  As  this 
precipitate  obscures  the  finishing  point  of  the  reaction  with  methyl 
orange,  it  is  perhaps  better,  in  such  cases,  to  add  an  excess  of  the 
standard  acid,  that  is  to  say,  to  continue  adding  acid  until  no  further 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  63 

precipitation  takes  place,  and  determine  the  excess  of  acid  in  the  fil- 
trate, or  a  definite  fraction  of  it,  by  titration  with  standard  alkali. 

In  this  case,  however,  the  whole  of  the  alkali  metal  of  the  double 
salts  will  be  determined  as  forming  part  of  the  total  alkali,  the 
probable  reactions  being 

K2ZnCy4  +  2HR  =  ZnCy2  +  2HCy  +  2KR, 
K2Cu2Cy4  +  2HR  =  Cu2Cy2  +  2HCy  +2KR, 
KAgCy2  -f  HR  =  AgCy  +  HCy  +  KR, 
R  being  any  negative  monovalent  radiate. 

(B)  ESTIMATION  OF  PROTECTIVE  ALKALI. 

Titration  with  standard  acid  and  phenol  phthalein  indicator, 
after  addition  of  silver  nitrate. 

This  method  was  devised  by  the  writer  in  1894,  and  is  fully  de- 
scribed in  the  Chemical  News,  Vol.  LXXL,  p.  93.  It  is  based  on 
the  facts  already  pointed  out:  (a)  That  double  cyanides  of  silver 
are  neutral  to  phenol  phthalein.  (&)  That  the  amount  of  acid  re- 
quired to  neutralize  the  hydrates  and  carbonates  towards  phenol 
phthalein  is  a  measure  of  protective  alkali  as  defined,  (c)  That 
the  presence  of  cyanide  or  double  cyanide  of  silver  does  not  interfere 
with  the  titration  of  alkali,  the  double  salt  not  being  decomposed 
until  the  whole  of  the  alkali  is  neutralized. 

The  principal  substances  included  in  this  test  are :  Hydrates, 
carbonates  (converted  in  the  titration  to  bicarbonates),  ammonia, 
a  small  portion  of  K2ZnCy4,  zincates  (Zn(OK)2).  The  latter  prob- 
ably do  not  coexist  with  free  cyanides. 

The  method  of  testing  is  as  follows :  Silver  nitrate  is  added  to  a 
measured  volume  of  the  solution  until  a  permanent  turbidity  is 
produced.  The  ordinary  standard  solution  may  be  used,  so  that 
the  same  measured  portion  of  the  liquid  to  be  tested  will  serve  for 
the  determination  of  both  cyanide  and  protective  alkali.  This 
latter  is  a  great  advantage  of  the  method  in  practical  work,  and 
enables  it  to  be  applied  constantly  for  the  control  of  daily  operations 
in  the  cyanide  plant,  the  test  being  so  simple  as  to  be  well  within 
the  capacity  of  the  "average  man  on  shift."  Addition  of  even  a 
considerable  quantity  of  silver  in  excess  of  the  necessary  amount 
does  not  materially  affect  the  result.  A  drop  of  the  alcoholic  0.5  per 
cent  phenol  phthalein  solution  is  now  added  to  the  turbid  liquid  in 
the  flask  (without  filtering),  and  the  resulting  pink  fluid  is  titrated 


64  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

with  N/10,  or  any  convenient  standard  acid,  until  the  color  entirely 
disappears.  The  amount  of  standard  acid  used  measures  the  pro- 
tective alkali. 

Influence  of  Zinc. — As  already  pointed  out,  only  a  small  portion 
of  K2ZnCy4  is  shown  as  alkali  when  titrated  with  acid  and  phenol 
phthalem.  Moreover,  the  addition  of  excess  of  silver  nitrate  after 
the  first  turbidity  causes  the  K2ZnCy4  solution  to  become  quite 
neutral  to  this  indicator  at  some  point  before  the  complete  precipi- 
tation of  the  cyanide.  When  large  quantities  of  zinc  are  present  (in 
absence  of  much  hydrate  or  carbonate  of  the  alkalis),  addition  of 
silver  nitrate,  as  recently  pointed  out  by  L.  M.  Green,  causes  the 
solution  to  become  acid  to  phenol  phthalem. 

METHOD  No.  2. 

Estimation  of  Protective  Alkali  after  Addition  of  Silver  Nitrate 
and  Potassium  Ferrocyanide. 

The  following  method,  devised  by  Leonard  M.  Green  (Proceed- 
ings Inst.  Min.  and  Met.,  October,  1901),  obviates  the  difficulties 
in  the  last  method  due  to  the  presence  of  zinc,  but  it  is  questionable 
whether  the  value  obtained  represents  the  protective  alkali  as  de- 
fined. (See  p.  6  above.) 

The  method  is,  briefly,  as  follows:  The  total  cyanide  is  first 
determined  by  titration  with  silver  nitrate,  using  the  alkaline  iodide 
indicator..  Another  portion,  say,  50  c.c.,  of  the  original  solution  is 
now  taken,  an  excess  of  ferrocyanide  solution  added,  and  then  a 
little  more  silver  solution  than  was  used  in  the  previous  test,  to 
ensure  the  complete  conversion  of  all  cyanides  into  silver  salts. 
Phenol  phthalem  is  then  added  and  the  liquid  titrated  with  standard 
acid  as  in  the  previous  method.  We  may  suppose  the  reactions  (in 
absence  of  alkalis  and  ferrocyanides)  to  be  as  follows,  white  zinc 
double  cyanide  is  in  excess: 

(a)     K2ZnCy4  +  AgNO,  =  ZnCy2  +  KAgCy2  +  KN03 ; 
but  in  presence  of  a  sufficient  excess  of  silver  nitrate, 

(6)     K2ZnCy4  +  2AgN03  =  2KAgCy2  +  Zn(N03)2. 

Bettel    (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol.  L, 
p.  164)  gives  the  following  as  the  reaction  taking  place  when  excess 
of  ferrocyanide  is  added  to  a  solution  of  the  zinc  double  cyanide : 
3K,ZnCy4  +  2K4FeCy8  =  K2Zn3(FeCya)  +  12KCy, 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  65 

so  that  in  presence  of  an  excess  of  AgN03  the  solution  would  be- 
come neutral  by  conversion  of  KCy  into  KAgCy2. 

In  absence  of  ferrocyanide  the  acidity  to  phenol  phthalei'n  may 
be  due  to  the  presence  of  zinc  nitrate,  which  would  neutralize  any 
alkaline  hydrate  or  carbonate  as  follows : 

Zn(N03)2  +  2KOH  =  Zn(OH)2  +  2KN03, 
Zn(N03)2  -f-  K2C03  =  ZnC03  -f  2KN03. 
On  addition  of  ferrocyanide  the  alkali  is  regenerated  as  follows : 
K4FeCy6  +  Zn(OH)2  =  Zn2FeCy6  +  2KOH 
K4FeCy6  +  ZnC03  =  Zn2FeCye  +  K2C03. 
Green  gives  the  reaction  between  zinc  nitrate  and  alkaline  car- 
bonate as  follows,  the  zinc  being  supposed  to  form  a  basic  carbonate : 
2Zn(N03)2  +  3K2C03  -f  2H20= 
Zn(OH)2-ZnC03  +  2KHC03  +  4KN03, 
and  the  corresponding  reaction  on  addition  of  ferrocyanide : 
Zn(OH)2  ZnC03  +  K4FeC6  = 
Zn2FeCy6  +  2KOH  4-  K2C03, 

a   double   ferrocyanide  of   doubtful   composition   being   probably 
formed,  so  that  the  equation  assumes  the  form : 
Zn(OH)2-ZnC03  +  zK4FeCy6  = 
Zn2FeCy6-(z-t)K4FeCye  -f  2KOH  +  K2CO,. 

METHOD  No.  3. 

Estimation  of  Protective  Alkali  by  Neutralizing  with  Potassium 
Bisulphate  and  Distilling. 

A.  F.  Crosse  (Proceedings  Chem.  and  Met.  Soc.  South  Africa) 
describes  the  following  process :  500  c.c.  of  the  solution  are  taken, 
1  gram  of  potassium  bisulphate  (KHS04)  added,  and  the  liquid 
boiled  in  a  retort  with  condenser  for  45  minutes.  The  hydrocyanic 
acid  distilled  over  is  collected  in  caustic  potash,  and  the  resulting 
liquid  titrated  in  the  usual  way  with  silver  nitrate  and  potassium 
iodide.  The  reactions  are  as  follows: 

KHS04  +  KCy  =  K2S04  +  HCy. 

When  alkali  is  present,  a  portion  of  the  bisulphate  is  neutralized : 
KHS04  +  KOH  =  K2S04  +  H20, 
KHS04  +  K2C08  =  K2S04  -f  KHCO,, 

and  the  amount  of  hydrocyanic  acid  given  off  is  proportionately  less, 
From  the  first  equation  it  is  evident  that  in  the  absence  of 


66  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

protective  alkali,  1  gram  of  KHS04  liberates  0.1985  gram  of  HCy. 

The  difference  between  the  cyanide  equivalent  of  the  bisulphate 
used  and  the  cyanide  actually  found  in  the  distillate  is  the  cyanide 
equivalent  of  the  protective  alkali,  which  may  be  converted  into 
terms  of  KOH  by  multiplying  by  the  factor  56/65  =  0.8615. 

An  excess  of  free  potassium  cyanide  must  always  be  present,  hence 
it  is  recommended  to  add  2  grams  of  chemically  pure  potassium 
cyanide  before  distilling.  It  is  obvious,  also,  that  the  bisulphate 
must  be  added  in  excess  of  the  amount  required  to  neutralize  the 
protective  alkali. 

While  this  method  may  possibly  give  accurate  results  with  careful 
working,  it  is  certainly  very  slow  and  cumbrous,  and  hardly  adapted 
to  the  requirements  of  practical  work.  It  is  also  probable  that  by 
boiling  the  cyanide  solution  for  45  minutes  some  decompositions 
would  occur  which  would  vitiate  the  result. 

In  the  absence  of  sufficient  free  cyanide  it  is  possible  that  a  part 
of  the  bisulphate  would  be  consumed  in  precipitating  zinc  cyanide, 

K2ZnCy4  +  2KHS04  =  ZnCy2  +  2HCy  -f  2K2S04, 
but  this  would  introduce  no  error,  as  an  equivalent  of  HCy  is  liber- 
ated. 

(C,  D)    ESTIMATION  OF  HYDRATES,  CARBONATES  AND 

BlCARBONATES. 

The  tests  for  these  substances  may  conveniently  be  considered 
together,  as  most  of  the  methods  for  estimating  any  one  of  them 
involves  a  determination  of  one  of  the  others.  The  behavior  of  the 
various  bodies  individually  towards  different  indicators,  when  neu- 
tralized by  dilute  acid,  has  already  been  described.  It  must  be  here 
remarked  that  hydrates  and  bicarbonates  cannot  coexist  in  the  solu- 
tion, as  the  reaction, 

KOH  +  KHC03  =  K2C03  +  H20, 

or  its  equivalent,  would  immediately  occur ;  hence  we  have  to  con- 
sider only  the  cases  in  which  cyanides  are  present  together  with 
(a)  hydrates  and  monocarbonates,  (6)  monocarbonafes  and  bicar- 
bonates. 

(a)  Solution  Contains  Cyanides,  Hydrates  and  Monocarbonates. 
— From  what  has  been  said  above  (tests  for  total  and  protective  al- 
kali) it  is  clear  that  the  equivalent  of  "hydrate  -f-  |  monocarbonate' 
may  be  fonnd  by  titrating  with  standard  acid  and  phenol  phthalei'n 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  67 

after  addition  of  silver  nitrate  to  produce  turbidity ;  and  the  equiva- 
lent of  'cyanide  -f  hydrate  +  monocarbonate'  by  direct  titration  of 
another  portion  of  the  solution  with  standard  acid  and  methyl 
orange  (without  adding  AgN03). 

The  cyanide  may  be  found  by  any  of  the  ordinary  methods,  and  its 
alkali  equivalent  deducted  from  the  last  result,  KCy  £  0.8615  = 
KOH.  We  have  then  (A)  =  hydrate  -f  \  carbonate.  (B)  =  hy- 
drate -f-  carbonate  —  whence  hydrate  =  2 A — B,  and  carbonate  = 
2(B-A). 

(&)  Solution  Contains  Cyanides,  Carbonates  and  Bicarbonates. 
— When  no  hydrate  is  present  the  titration  with  acid  and  phenol 
phthalem  after  addition  of  AgN03  gives  us  simply  '•£  carbonate/ 
while  the  direct  titration  with  acid  and  methyl  orange  gives 
'cyanide  -f-  carbonate  -f-  bicarbonate.' 

Deducting  cyanide  as  before,  we  have  (A)=%  carbonate 
(B)  =  carbonate  +  bicarbonate  —  whence  carbonate  =  2 A,  and  bi- 
carbonate =  B — 2A. 

In  presence  of  zinc  double  cyanide  and  ammonia  further  com- 
plications arise,  and  this  method  cannot  be  applied. 


METHOD  No.  2. 

Estimation  of  Hydrates,  Carbonates  and  Bicarbonates  by  Combined 
Titrations,  with  and  without  Addition  of  Barium  Salts. 

(A)  Hydrates  may  be  estimated  by  adding  the  necessary  amount 
of  a  standard  solution  of  barium  chloride  to  precipitate  the  car- 
bonate (avoiding  an  excess,  which  would  tend  to  precipitate  a  por- 
tion of  hydrate),  filtering  and  titrating  the  filtrate,  or  a  measured 
fraction  of  it,  with  standard  acid  and  methyl  orange.     The  result 
gives  'hydrate  -f-  cyanide,'  from  which  the  hydrate  is  obtained  by 
deducting  the  equivalent  of  cyanide. 

The  reactions  are: 

BaCL  +  K2CO,  =  2KC1  +  BaC03. 

BaCl2  +  2KHC03  =  2KC1  +  H20  -f  C02  +  BaC08. 

(B)  The  precipitated  barium  carbonate,  after  thorough  washing 
in  boiling  distilled  water,  is  titrated  with  standard  acid  and  methyl 
orange,  which  gives  the  equivalent  of  "carbonate  -f  £  bicarbonate." 

(C)  By  adding  an  excess  of  alkaline  hydrate  free  from  C02 


68  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

before  addition  of  barium  chloride,  the  whole  of  the  carbonate  and 
bicarbonate  may  be  precipitated  as  BaC08 : 

KHC03  +  KOH  =  H20  -f  K2C08, 
BaCl2  -f-  K2C03  =  2KC1  +  BaC03. 

If  the  solution  be  now  filtered  and  tested  as  before,  the  difference 
between  the  amount  of  hydrate  added  and  the  amount  found  after 
precipitation  with  barium  chloride  gives  the  equivalent  of  the  bi- 
carbonate, and  titration  of  the  washed  precipitate  gives  the  equiva- 
lent of  'carbonate  +  bicarbonate/  (For  further  details  see  Sut- 
ton,  Volum.  Anal.,  8th  ed.,  pp.  60-63.) 

Modification  in  Presence  of  Zinc. — When  zinc  is  present,  Green 
modifies  this  process  as  follows  (.Proceedings  Inst.  Min.  and  Met., 
October,  1901) :  An  excess  of  barium  chloride  is  first  added  to  the 
solution  (sufficient  to  precipitate  the  sulphates  and  carbonates), 
then  excess  of  potassium  ferrocyanide,  then  twice  the  amount  of 
silver  nitrate  necessary  to  indicate  total  cyanide,  viz.,  sufficient 
to  precipitate  the  whole  of  the  cyanide.  A  slight  excess  does  not 
matter,  as  it  merely  precipitates  some  chloride,  or,  if  no  chlorides 
are  present,  some  sulphocyanide  or  ferrocyanide.  The  zinc  all  occurs 
in  the  precipitate  as  ferrocyanide,  and  the  alkaline  hydrates  are  left 
in  solution.  (Barium  chlorile  being  present,  the  carbonates  are 
already  precipitated  as  BaC03.)  Phenol  phthalein  is  now  added, 
and  the  solution  titrated  with  N/10  nitric  acid  till  colorless,  or  till 
it  acquires  the  faint,  greenish-yellow  tinge  produced  by  the  excess 
of  ferrocyanide. 

Experiments  which  I  have  made  with  this  process  gave  only  mod- 
erately good  results,  the  numbers  being  generally  less  than  theoret- 
ical, perhaps  owing  to  precipitation  of  some  barium  hydrate. 

(E)     ESTIMATION  OF  AMMONIA. 

Free  ammonia  and  the  carbonates  and  bicarbonates  of  ammonium 
are  alkaline  to  methyl  orange,  and  are  estimated  along  with  the 
other  ingredients  in  titrating  total  alkali  (see  above) .  Ammonium 
chloride  is  neutral  to  all  indicators. 

Bettel  (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol.  I.,  p. 
168)  gives  the  following  method  for  estimating  ammonia  in  cyanide 
solutions:  "If  sufficient  nitrate  of  silver  be  added  to  a  solution  (say, 
10  c.c.)  to  wholly  precipitate  the  cyanogen  compounds,  and  a  drop 
or  two  of  normal  hydrochloric  acid  be  then  added,  the  whole  made 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  69 

up  to  100  c.c.  and  shaken,  then  filtered,  and  10  c.c.  of  the  filtrate 
distilled  with  150  c.c.  water  from  a  tubulated  flask,  and  the  steam 
condensed  in  a  Liebig's  condenser,  the  ammonia  coming  over  may 
be  readily  estimated  by  color  test  with  Nessler  solution,  and  com- 
parison with  distilled  water  free  from  ammonia  and  standard  am- 
monium chloride  solution  containing  0.01  gram  NH5  per  liter, 
treated  with  Nessler  solution." 

If  the  ammonia  in  combination  as  ammonium  salts  is  to  be  esti- 
mated, sodium  carbonate  must  be  added  to  the  liquid  to  be  dis- 
tilled. (For  further  details  as  to  distillation  and  colorimetric  test, 
see  Sutton,  Volum.  Anal,  8th  edv  pp.  447,  456.) 


70  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

CLASS  III. 
EEDUCING  AGENTS. 

General  Remarks.— The  efficiency  of  cyanide  solutions  for  gold 
extraction  depends,  as  already  pointed  out,  not  only  on  the  amount 
of  free  cyanide  present,  but  also  on  the  amount  of  oxygen.  Hence 
any  substance  which  is  capable  of  absorbing  oxygen  will  exercise  an 
injurious  effect,  and  it  is  sometimes  desirable  to  be  able  to  determine 
the  amount  of  such  deoxidizing  or  reducing  agent. 

We  shall  first  describe  a  general  method  for  estimating  the  amount 
of  reducing  agent,  irrespective  of  its  actual  composition,  and  after- 
wards discuss  the  methods  in  use  for  determining  the  amount  of  the 
various  individual  bodies  commonly  met  with  in  cyanide  solutions, 
which  exercise  a  reducing  influence. 

This  part  of  the  subject  will  be  considered  under  the  following 
heads: 

(A)  Eeducing  Power. 

(5)  Ferrocyanides, 

(C)  Sulphocyanides  (Thiocyanates). 

(D)  Sulphides. 

(E)  Other  Eeducing  Agents,  Such  as  Nitrites,  Formates,  etc. 


SECTION  A. 
(1)  ESTIMATION  OF  EEDUCING  POWEB. 

A  rough  idea  of  the  relative  quantities  of  reducing  agent  in  vari- 
ous solutions  may  be  obtained  by  comparing  the  amounts  of  standard 
permanganate  required  to  give  a  permanent  tint  to  equal  volumes  of 
the  different  solutions,  tested  under  the  same  conditions,  and  acidi- 
fied in  each  case  with  sulphuric  acid. 

Pure  potassium  cyanide  and  other  simple  cyanides,  when  treated 
in  this  way,  give  hardly  any  reaction  with  permanganate,  a  single 
drop  of  N/10  KMn04  added  to  the  acidulated  cyanide  solution 
being  generally  sufficient  to  establish  a  permanent  pink  tint,  owing 


CHEMISTBY    OF    CYANIDE    SOLUTIONS.  71 

to  the  fact  that  the  hydrocyanic  acid  liberated  scarcely  affects  the 
permanganate  at  all.  When  ferrocyanides,  sulphocyanides,  sul- 
phides, nitrites,  formates,  and  some  other  substances  are  present, 
however,  the  pink  color  at  first  disappears  rapidly  as  each  drop  is 
added.  Towards  the  finish  the  disappearance  is  sometimes  not 
sharp,  the  color  fading  when  the  solution  is  left  standing  for  a  few 
moments. 

Indirect  Estimation  by  Adding  Excess  of  Permanganate.  —  When 
the  finishing  point  is  indefinite,  it  is  perhaps  better  to  determine  the 
reducing  power  by  adding  to  the  acidulated  solution  a  moderate 
excess  of  permanganate,  allowing  to  stand  for  some  time,  then  add- 
ing an  excess  of  potassium  iodide  to  the  pink  liquid  until  the  color 
changes  to  brownish-yellow;  in  this  reaction  iodine  is  liberated  in 
proportion  to  the  excess  of  permanganate  present.  The  iodine  is 
then  immediately  determined  by  means  of  standard  thiosulphate, 
using  starch  indicator.  As  the  titration  is  performed  in  an  acid 
liquid,  a  starch  solution  prepared  with  addition  of  caustic  soda  may 
conveniently  be  employed;  this  will  keep  for  a  long  time,  whereas 
the  ordinary  starch  indicator  must  always  be  freshly  prepared. 
Deducting  the  equivalent  of  the  iodine  thus  found,  in  terms  of 
standard  permanganate,  from  the  total  amount  of  permanganate 
added,  we  have  the  quantity  of  permanganate  which  corresponds  to 
the  reducing  agents  present  in  the  solution  under  examination. 

In  this  process  it  is  assumed  that  hydrocyanic  acid  has  no  action 
on  iodine.  The  same  assumption  is  made  in  the  method  for  esti- 
mating cyanogen  bromide,  to  be  noticed  below.  It  is  possible,  how- 
ever, that  some  iodide  of  cyanogen  (ICy)  may  be  formed  according 
to  the  reaction 


but  this  would  introduce  no  error,  as  the  cyanogen  iodide  is  com- 
pletely decomposed  by  the  thiosulphate. 

The  permanganate  may  be  standardized  by  adding  excess  of 
potassium  iodide  to  a  measured  volume  and  determining  the  amount 
of  thiosulphate  required  to  destroy  the  color. 

The  starch  indicator  should  be  added  near  the  finish,  after  suffi- 
cient standard  thiosulphate  has  been  run  in  to  nearly  discharge 
the  original  yellow  color  of  the  iodine.* 


*  Instead  of  using  potassium  Iodide  and  thiosulphate,  the  excess  of  perman- 
ganate might  be  determined  by  means  of  standard  oxalic  acid.  When  the  amount 
of  reducing  agent  is  small,  N/100  solutions  of  permanganate,  etc.,  should  be 


72  CHEMISTRY    OF    CYANIDE    SOLUTION'S. 

The  standard  solutions  which  may  be  conveniently  used  are: 
(1)  Potassium  permanganate,  KMn04,  3.16  grams  per  liter  (deci- 
normal  solution).  (2)  Sodium  thiosulphate,  Na2S203-5H20, 
24.8  grams  per  liter  (decinormal  solution).  (3)  Potassium  iodide, 
KI,  16.6  grams  per  liter.  (The  solid  salt  may  be  used  instead  of 
the  latter  solution.) 

The  probable  reactions  are: 

(a)  5KI  +  KMn04 -f- 4H2S04  = 

MnS04  +  3K2S04  -f  43^0  +  51. 
(&)  Na2S203  +  I2  =  2NaI  +  Na2S408. 

Definition  of  Beducing  Power. — For  the  sake  of  precision  in  com- 
parative tests  we  may  define  the  'reducing  power*  of  a  solution  as 
the  number  of  cubic  centimeters  of  N/10  permanganate  (3.10 
grams  per  liter)  which  must  be  added  to  give  a  permanent  coloration 
with  1  c.c.  of  the  solution  to  be  examined,  a  sufficient  amount  of 
free  sulphuric  acid  being  present  in  every  ease. 

(2)  ESTIMATION  or  OXIDIZABLE  ORGANIC  MATTER. 

Bettel  (Proceedings  Chem.  and  Met.  Soc.  South  Africa,  Vol. 
I.,  p.  166)  gives  a  method  somewhat  similar  to  that  just  described, 
as  follows :  "In  treating  spruit  tailings,  or  material  containing  de- 
caying vegetable  matter,  I  offer  the  following  method  for  testing 
colored  solutions:' 

"(a)  Prepare  a  solution  of  a  sulphocyanide  so  that  1  c.c.  sulpho- 
cyanide  =  1  c.c.  N/100  KMn04. 

"(&)  To  50  c.c.  of  the  liquid  to  be  tested  add  sulphuric  acid  in 
excess,  and  then  a  large  excess  of  N/100  permanganate.  Keep  at 
60°-70°  for  an  hour,  then  cool  and  titrate  back  with  the  sulpho- 
cyanide solution. 

"Kesult:    • 

02  consumed  in  oxidizing  organic  matter  -f- 
02  consumed  in  oxidizing  K4FeCy6  -f- 
02  consumed  in  oxidizing  KCNS. 

"After  estimating  KCNS  and  K4FeCy6,  a  simple  calculation  gives 
the  oxygen  to  oxidize  organic  matter.  The  result,  multiplied  by 
9  will  give,  approximately,  the  amount  of  organic  matter  present." 

In  order  to  prepare  another  portion  of  the  solution  for  the  esti- 
mation of  ferrocyanides  and  sulphocyanides,  he  proceeds  as  follows : 
The  solution  is  shaken  up  with  powdered  quicklime  and  filtered, 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  73 

it  is  then  "of  a  faint  straw  color,  and  is  in  a  proper  condition  for 
analysis.  In  such  clarified  solution  the  oxidizable  organic  matter 
is  no  longer  present  and  the  ferro  and  sulphocyanogen  estimations 
are  readily  performed/' 


SECTION  B. 
ESTIMATION  OF  FERROCYANIDES. 

A  very  large  number  of  methods  have  been  used  or  proposed  for 
the  estimation  of  ferrocyanides,  both  in  cyanide  solutions  and  in 
various  commercial  products  in  which  they  occur  mixed  with  other 
bodies.  These  processes  may  be  classified  as  follows: 

(A)  Those  depending  on  the  conversion  of  ferrocyanide  into  fer- 
ricyanide  in  presence  of  free  acid  by  the  action  of  KMn04. 

(B)  Those  depending  on  the  precipitation  of  ferrocyanide  as 
Prussian  blue  by  means  of  ferric  salts. 

(C)  Those  depending  on  the  reactions  of  ferro-  or  ferricyanides 
with  copper  salts. 

(Z?)  Those  depending  on  the  reactions  of  ferrocyanides  with  zinc 
salts. 

(E)  Those  depending  on  the  estimation  of  the  iron  after  com- 
plete decomposition  of  the  ferrocyanogen  by  means  of  strong  acids. 

GROUP    (A). — ESTIMATION  OF  FERROCYANIDES  BY  CONVERSION 

INTO  FERRICYANIDES  IN  ACID  SOLUTION  BY  MEANS  OF 

PERMANGANATE. 

METHOD  No.  1. 
Direct  Titration  of  the  Solution. 

This  process,  due  to  de  Haen,  is  described  in  Fresenius  (Quant. 
And.,  7th  edv  Vol.  I.,  p.  378)  and  Sutton  (Volum.  Anal.,  8th 
ed.,  p.  226). 

In  all  cases  where  the  solution  contains  no  other  reducing  agent 
the  amount  of  ferrocyanide  may  be  estimated  with  tolerable  ac- 
curacy by  this  method,  since,  as  pointed  out  above,  the  presence  of 


74  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

cyanides,  which  are  converted  on  acidulating  into  hydrocyanic 
acid,  does  not  interfere.  When  much  ferrocyanide  is  present  the 
solution  must  be  considerably  diluted  with  water,  otherwise  the  end- 
point  is  not  sharp.  About  100  c.c.  of  water  should  be  added  for 
every  0.1  gram  of  ferrocyanide  present. 

A  N/10  solution  of  ferrocyanide  contains  42.2  grams  per  liter, 
corresponding  volume  for  volume  with  N/10  permanganate  contain- 
ing 3.16  grams  KMn04  per  liter.  In  practice  N/20  or  N/100 
solutions  may  be  more  conveniently  used. 

The  finishing  point  is  shown  by  the  change  from  yellow  to  red- 
dish-yellow. Ferric  chloride  may  also  be  used  as  an  external  in- 
dicator; the  reaction  is  considered  to  be  complete  when  a  drop  of 
the  liquid  on  a  white  plate  no  longer  gives  a  blue  tinge  with  a  drop 
of  ferric  chloride. 

The  reaction  of  permanganate  on  ferrocyanide  in  presence  of 
sulphuric  acid  may  be  represented  as  follows: 
KMn04  +  5K4FeCye  +  4H2S04  = 

3K2S04  +  MnS04  +,  4H20  +  5K3FeCy6. 

METHOD  No.  2. 

Precipitation  of  Ferrocyanide  as  Prussian  Blue,  and  Subsequent 
Solution  in  Alkalis  and  Titration  with  Permanganate. 

This  modification  of  de  Haen's  method  was  introduced  by  Erlen- 
meyer  for  the  purpose  of  separating  sulphocyanides,  and  other  re- 
ducing agents  from  the  ferrocyanide,  before  titration  with  per- 
manganate (Emil  Erlenmeyer,  Wagner's  Jahresb&richt,  1860, 
p.  223).  Thorpe  (Diet,  of  Applied  Chemistry,  Art.  'Cyanide,'  p. 
631)  states  that  of  the  many  methods  proposed,  Erlenmeyer's  is 
probably  the  most  exact. 

The  ferrocyanogen  is  precipitated  by  an  excess  of  ferric  chloride 
in  the  presence  of  an  excess  of  hydrochloric  acid,  as  Prussian  blue, 
which  he  filters  off,  washes  and  decomposes  by  means  of  caustic  pot- 
ash. 

(a)  3K,FeCy.'+  2Fe2Cl8  =  Fe4(FeCy6)8  =  12KC1. 

(6)  Fe4(FeCy6)3  +  12KOH  =  3K4FeCye  +. 2Fe2(OH)6. 

The  ferric  hydrate  is  then  filtered  off,  and  the  ferrocyanogen  de- 
termined in  the  acidified  filtrate  by  titration  with  KMn04  accord- 
ing to  de  Haen's  method. 

With  regard  to  this  process  Bettel  remarks  (Proceedings  Chem. 


DHEMISTEY  OF  CYANIDE  SOLUTIONS.  75 

and  Met.  Soc.  South  Africa,  Vol.  I.,  p.  220),  "the  decomposition 
of  the  Prussian  blue  by  alkali  and  subsequent  titration  of  the  ferro- 
cyanide  formed  is  inaccurate  on  account  of  the  incomplete  decom- 
position of  the  ferric  ferrocyanide  by  potash."  Bettel  suggests  the 
following  method. 

METHOD  No.  3. 

Estimation  of  Ferrocyanide  by  Addition  of  the  Solution  to  be 
Tested  to  a  Known  Amount  of  Acid  Permanganate.* 

In  the  absence  of  organic  matter,  an  acidified  solution  of  a 
simple  cyanide  such  as  KCy  or  of  a  double  cyanide  (as  K2ZnCy4), 
i.e.,  a  solution  of  HCy,  is  not  affected  by  dilute  permanganate. 

In  presence  of  zinc,  the  method  is  as  follows : 

(a)  A  burette  is  filled  with  the  cyanide  solution  to  be  tested, 
and  the  liquid  run  into  10  or  20  c.c.  of  N/100  permanganate, 
strongly  acidified  with  H2S04  until  the  color  is  just  discharged. 
Result  =  A  (calculated  for  50  c.c.  of  original  solution). 

(&)  A  solution  of  ferric  sulphate  or  chloride  is  acidified  with 
sulphuric  acid  and  50  c.c.  of  the  cyanide  solution  poured  in. 
After  shaking  for  about  half  a  minute,  the  Prussian  blue  is  separated 
from  the  liquid  by  filtration  and  the  precipitate  and  filter  paper 
washed.  The  filtrate  is  next  titrated  with  N/100  KMn04.  Re- 
sult =  B.  Equivalent  of  ferrocyanide  =  A — B. 

In  this  process  the  first  titration  estimates  all  the  reducing  agents, 
and  the  second,  all  except  ferrocyanides,  hence  the  difference  of  the 
two  gives  the  ferrocyanide. 

METHOD  No.  4. 

Estimation  of  Ferrocyanide  by  Preliminary  Oxidation  to  Ferri- 
cyanide,  then  Boiling  with  Alkali  and  Ferrous  Salt  to 
Convert  Again  into  Ferrocyanide,  Finally  Titrat- 
ing with  Permanganate. 

J.  Tcherniac  (Chemical  News,  Vol.  XLVIL,  p.  254,  abstract 
from  paper  in  Zeitschrift  f.  analyt.  Chem.  1883)  gives  the  fol- 
lowing method: 

The  solutions  required  are :  (a)  A  saturated  solution  of  potassium 
permanganate;  (&)  A  standard  permanganate  solution  of  which 

*  Proceedings  Chem.  and  Met.  Soc.  South  Africa. 


76  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

1  c.c.  corresponds  to  0.1  gram  K4FeCy6;  (c)  A  solution  of  ferrous 
sulphate  containing  50  to  100  grams  per  liter. 
The  method  as  originally  described  is  as  follows : 
A  measured  quantity  of  the  solution  to  be  tested,  containing 
about  3  grams  ferrocyanide,  is  made  up  to  500  c.c.  with  sulphuric 
acid  in  a  thin  measuring  flask,  and  mixed  with  so  much  of  the 
saturated  permanganate  that  the  red  color  does  not  disappear  on 
agitation  for  some  minutes.  The  whole  is  allowed  to  stand  for  about 
half  an  hour  (for  complete  oxidation  of  ferrocyanides,  thiocyanates, 
sulphides,  etc.) ;  after  the  oxidation  is  complete,  caustic  soda  is 
added  in  large  excess,  and  the  whole  is  heated  to  a  boil,  with  con- 
stant stirring.  The  hot  solution  is  mixed  with  so  much  ferrous  sul- 
phate that  the  precipitate  is  colored  black  by  the  ferroso-ferric  oxide 
liberated,  allowed  to  cool,  filled  up  to  the  mark  again,  and  filtered. 
In  an  aliquot  part  of  the  filtrate  (say,  250  c.c.)  the  quantity  of  fer- 
rocyanogen  re-formed  by  the  ferrous  sulphate  is  determined  by  titra- 
tion  with  the  standard  permanganate  after  acidulation  with  sul- 
phuric acid. 

This  method  is  described  by  Alfred  Adair  (Journal  Chem.  and 
Met.  Soc.  South  Africa,  Vol.  III.,  p.  140)  with  a  slight  modifica- 
tion, by  which  it  may  be  applied  for  the  estimation  of  cyanogen  in 
commercial  cyanide.  He  states  that  in  the  treatment  with  saturated 
permanganate  the  ferrocyanide  is  oxidized  to  ferricyanide  only, 
whereas  cyanates,  sulphocyanides  and  other  impurities  are  either 
destructively  oxidized,  or  converted  into  substances  which  do  not  in- 
terfere with  the  final  reduction  of  ferri-  to  ferrocyanide,  and  the 
titration  of  the  latter  with  permanganate.  He  also  states  that  esti- 
mations can  be  made  in  15  minutes,  and  that  concordant  results  are 
obtained  by  different  operators. 

Adair's  modification  of  Tcherniac's  method  is  as  follows : 
The  solutions  required  are:    (a)  25  per  cent  caustic  alkali;  (6) 
20  per  cent  H2S04,  pure;  (c)  saturated  solution  of  KMn04;  (d) 
saturated  solution  of  FeS04;  (e)  N/10  KMn04,  or,  more  conve- 
niently, a  solution  such  that  1  c.c.  =  0.100  gram  total  cyanogen. 

The  permanganate  solution  (e)  is  standardized  as  follows: 
Three  grams  of  potassium  ferrocyanide,  K4FeCye'3Ag,  are  dissolved 
in  300  c.c.  water,  and  15  c.c.  of  the  20  per  cent  H2S04  (6)  are 
added,  then  if  n  equals  number  of  c.c.  of  solution  (e)  consumed 

880  x  3  =  Value  of  Cy  in  grams  per  c.c. 
157.9  n  J 


CHBMISTEY    OF    CYANIDE    SOLUTIONS.  77 

(i)  Ten  grams  of  the  commercial  cyanide  are  weighed  into  a  liter 
flask,  and  about  200  c.c.  water  added  to  dissolve  it.  Add  2  c.c.  of 
the  alkali  solution  (a)  and  a  quantity  of  the  ferrous  sulphate  solu- 
tion (d)  equal  to  12  grams  FeS04-7Ag.  Add  the  latter  5  c.c.  at 
a  time,  and  shake  well. 

6KCy  +  FeS04  +  alkali  =  K4FeCy6  -f-  K2S04  -f  alkali. 

The  reaction  is  immediate. 

[This  step,  of  course,  would  be  omitted  where  the  method  is  ap- 
plied for  estimating  ferrocyanides  only.] 

(ii)  Add  sulphuric  acid  until  Prussian  blue  is  formed,  then 
15  c.c.  of  the  sulphuric  acid  solution  (&)  and  saturated  solution  of 
permanganate  (c),  until  the  color  remains  persistent;  the  color 
can  be  seen  through  the  edges.  An  excess  of  1  or  2  c.c.  or  more 
does  not  matter.  The  above  quantity  of  acid  is  enough  for  each 
gram  of  KMn04  added,  but  if  more  than  1  gram  of  KMn04 
is  used,  acid  must  be  added  in  the  same  proportion,  viz.: 
15  c.c.  to  each  gram  of  KMn04  used.  If  much  sulphocyanide  is 
present,  allow  to  stand  15  minutes,  and,  if  necessary,  a  further  addi- 
tion of  KMn04  may  be  made.  The  reaction  is  given  above  (under 
Method  No.  1). 

(Hi)  Next  add  ferrous  sulphate  solution  (d)  in  quantity  equal  to 
15  grams,  and  immediately  15  c.c.  alkali  solution  (a).    The  liquid 
must  be  strongly  alkaline.     Shake  thoroughly  and  make  up  to  the 
mark,  again  mixing  thoroughly.     The  reaction  is  : 
K3FeCy6  +  FeS04  +  3KOH  = 


[This  is  the  simplest  possibility,  but  it  may  be  2K3FeCya  -f 
4  =  Fe3(FeCy6)2  +  3K2S04  and  Fe3(FeCy6)2  -f  8KOH  = 
2K4FeCy6  +  Fe304-4H20.] 

(iv)  Filter  through  a  large  folded  filter.  The  titration  is  com- 
pleted by  taking  500  c.c.,  or  an  aliquot  portion,  adding  20  c.c. 
H2S04,  and  then  running  in  the  required  amount  of  the  standard 
KMn04  (e). 

The  influence  of  the  precipitate  on  the  results  is  small.  It  may 
be  ascertained  by  testing  a  weighed  portion  of  pure  K4FeCy6  -f- 
3H20,  adding  the  quantities  of  solutions,  as  for  an  impure  sample. 

Thorpe  (Diet,  of  Applied  Chemistry,  Art.  'Cyanide,'  p.  633) 
criticises  Tcherniac's  method  as  applied  to  the  determination  of 
actual  ferrocyanide  in  the  crude  product  obtained  in  the  process  of 
manufacture.  After  giving  an  outline  of  the  process  he  says: 


78  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

"Tcherniac  does  not  direct  us  to  maintain  a  red  color  in  the 
mixture  by  occasional  addition  of  more  permanganate,  but  he  no 
doubt  means  us  to  do  so.  At  last  the  mixture  is  made  strongly 
alkaline  by  addition  of  caustic  soda,  heated  to  boiling  and  mixed 
with  ferrous  sulphate  to  reconvert  the  red  into  yellow  prussiate 
with  formation  of  magnetic  oxide  of  iron,  which  is  filtered  off. 
The  filtrate  is  acidified,  and  its  prussiate  (i.e.,  ferrocyanide)  deter- 
mined by  titration  with  a  standard  solution  of  permanganate  (de 
Haen's  method).  The  method  obviously  is  based  on  the  presump- 
tion that  in  the  first  process  of  oxidation  the  thiocyanate  is  con- 
verted into  substances  not  susceptible  of  conversion  into  prussiate  by 
treatment  with  ferrous  hydrate  and  caustic  alkali ;  but  this  we  be- 
lieve is  a  mistake;  as  shown  by  Erlenmeyer,  permanganate  in  acid 
solution  converts  thiocyanate  into  cyanide  and  sulphate,  and  the 
HCy,  as  far  as  it  survives,  will  be  reported  as  so  much  prussiate.'* 
6KCy  +  Fe(OH)2  =  K4FeCy6  -f  2KOH. 

METHOD  No.  5. 

Precipitation  of  Ferrocyanide  with  Alcohol  and  Subsequent  Titra- 
tion with  Permanganate. 

J.  Tcherniac  (Zeitschrift  f.  Anal.  Chem.,  abstracted  in  Chemi- 
cal News,  Vol.  XLVIL,  p.  254),  operates  as  follows : 

"Ten  c.c.  of  the  solution  is  poured  into  70  c.c.  of  alcohol  of  95  per 
cent,  to  which  a  little  acetic  acid  has  been  previously  added.  The 
precipitated  ferrocyanide,  after  washing  with  alcohol  of  90  per  cent 
until  the  washings  are  colorless,  is  dried  at  100°  C.  on  the  filter,  dis- 
solved in  water,  and  titrated  with  permanganate  solution."  The 
method  would  probably  be  applicable  only  to  very  concentrated  so- 
lutions. 

GROUP  (B). — ESTIMATION  OP  FERROCYANIDES  BY  PRECIPITATION 
AS  PRUSSIAN  BLUE. 

Fresenius  (Quant.  Anal.,  7th  ed.,  Vol.  I.,  p.  380)  gives  the  fol- 
lowing method,  due  to  H.  Rheineck  (Chem.  Centralbl,  1871,  p. 
778),  which  may  possibly  be  of  use  in  certain  cases:  It  depends  on 
the  fact  that  when  a  solution  of  a  ferric  salt  (sulphate  or  chloride) 
is  added  with  vigorous  agitation  to  a  liquid  containing  ferrocyanide, 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  79 

no  matter  whether  a  mineral  acid  is  present  or  not,  at  first  a  clear 
blue  fluid  is  produced,  which  becomes  afterwards  turbid,  and  when 
all  the  ferrocyanogen  is  exactly  thrown  down  a  flocculent  precipitate 
of  Prussian  blue  appears  suspended  in  a  clear  colorless  fluid.  A 
certain  measure  of  the  liquid  under  examination  is  taken,  and  the 
same  measure  of  a  standard  solution  of  ferrocyanide  of  potassium; 
to  these  are  added  a  solution  of  ferric  chloride  from  a  burette  till  the 
flocculent  precipitate  separates. 

In  the  case  of  alkaline  liquids  the  solution  must  be  first  acidu- 
lated with  dilute  H2S04  or  HN03.  If  thiocyanates  be  present,  the 
least  excess  of  iron  solution  will  cause  the  liquid  to  assume  a  deep 
red  color.  The  change  from  blue  to  red  furnishes  a  very  definite 
end-reaction. 


GBOUP    (C). — ESTIMATION   OF   FERBOCYANIDES  BY  EEACTIONS 
WITH  COPPER  SALTS. 

METHOD  No.  1. 

Estimation  by  Titration   with    Copper   Sulphate  Using   Ferric 
Chloride  as  Indicated. 

Fresenius  (Quant.  Anal,  7th  ed.,  Vol.  I.,  p.  380)  describes  the 
following  method,  due  to  E.  Bohlig  (Polytechn.  Notizblatt,  pp.  16, 
81),*  as  accurate  enough  for  technical  purposes.  The  standard  solu- 
tions required  are  1  per  cent  copper  sulphate,  CuS04  5H20  and  0.4 
per  cent  potassium  ferrocyanide,  K4FeCy6-3H20.  The  solution  to 
be  tested  is  titrated  with  the  standard  copper  solution  after  acidify- 
ing with  dilute  sulphuric  acid,  until  a  strip  of  filter  paper  dipped 
into  the  mixture  no  longer  becomes  blue  when  a  drop  of  ferric 
chloride  is  put  on  it. 

Any  sulphide  present  in  the  liquid  must  first  be  removed  by  boil- 
ing with  lead  carbonate  and  filtering. 

The  method  depends  on  the  precipitation  of  the  brownish-red  ferro- 
cyanide of  copper. 

2CuS04  -}-  K4FeCy6  =  SKjSC^  -f  Cu2FeCy6. 

For  standardizing  the  copper  solution  about  0.2  gram  of  ferro- 
cyanide dissolved  in  50  c.c.  of  water  may  be  used. 

My  experiments  with  this  method  show  that  while  cyanides  and 

*  See  also  Wagner.  Jahresbericht,  1861,  p.  237. 


80  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

ithiocyanates  do  not  appreciably  interfere,  the  presence  of  zinc 
double  cyanide  renders  the  method  useless,  as  a  part  (and  in  some 
cases  the  whole)  of  the  ferrocyanogen  is  precipitated  as  ferrocyanide 
of  zinc,  and  is  not  determined  in  the  titration. 


METHOD  No.  2. 

Titration  with  Copper  Sulphate  after  Adding  Ferric  Chloride,  Fil- 
tering off  Prussian  Blue  and  Treating  with  Alkali. 

0.  Knublauch  (Journal  Soc.  Chem.  Ind.,  Vol.  VIII.,  p.  732, 
1889)  gives  a  method  originally  intended  for  the  estimation  of  ferro- 
cyanogen in  spent  material  from  gas  purification,  but  which  may  be 
adapted  for  the  analysis  of  cyanide  solutions.  He  finds  that  the 
previous  method  (direct  titration  with  copper  sulphate)  is  affected 
6y  impurities  which  either  react  with  the  copper  or  impair  the 
delicacy  of  the  end-reaction. 

The  method  proposed  is  as  follows:  A  measured  volume  of  the 
liquid  to  be  tested,  say,  100  c.c.,  is  run  into  a  sufficient  excess  of  hot 
ferric  chloride  containing  hydrochloric  acid.  (The  solution  recom- 
mended contains,  per  liter,  60  grams  ferric  chloride  and  200  c.c. 
of  hydrochloric  acid  sp.  gr.  1.19.)  The  mixture  is  then  filtered  at 
80°  C.  through  a  folded  filter,  covered  over,  in  a  hot-water  funnel, 
and  the  precipitate  washed  rapidly  with  hot  water.  The  filter  paper 
with  the  precipitate  is  then  spread  in  a  porcelain  dish  and  treated 
with  20  c.c.  of  10  per  cent  caustic  potash,  taking  care  that  none  of 
the  Prussian  blue  escapes  decomposition.  The  liquid  is  then  fil- 
tered, and  if  free  from  sulphides  it  may  be  titrated  at  once,  after 
acidifying  with  sulphuric  acid.  In  each  test  2.5  to  5  c.c.  of  sul- 
phuric acid  (1:5)  are  used. 

If  sulphides  are  present,  the  liquid  must  first  be  treated  with  1  or 
2  grams  of  lead  carbonate  and  filtered. 

The  copper  sulphate  solution  used  contains  12  to  13  grams  per 
liter,  and  is  standardized  by  titrating  with  a  pure  ferrocyanide  solu- 
tion containing  4  grams  per  liter. 

The  finishing  point  may  be  found  either  by  'spot  test'  or  'filter 
test/ 

In  the  spot  test,  a  little  of  the  liquid  is  taken  out  on  a  glass  rod 
and  placed  on  a  filter  paper.  Ordinary  white  filter  paper  generally 
contains  sufficient  iron  to  give  a  blue  color  so  long  as  ferrocyanide  is 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  81 

in  excess,  or  a  drop  of  very  dilute  ferric  chloride  may  be  placed  on  the 
paper  near  the  spot,  so  that  the  reaction  is  observed  at  the  contact  of 
the  two  liquids.  When  the  blue  color  ceases  to  be  formed,  the  titra- 
tion  is  complete. 

In  the  filter  test  a  portion  of  the  liquid  is  carefully  filtered 
through  a  minute  filter,  and  the  clear  liquid  tested  with  ferric  chlo- 
ride ;  this  is  the  more  delicate  test  and  always  registers  slightly  higher 
than  the  spot  test. 

Care  must  be  taken  to  always  use  the  same  proportion  of  acid  in 
the  titration,  and  to  allow  in  all  cases  the  same  time  for  the  copper 
sulphate  to  react,  and  for  the  blue  color  to  appear  in  testing  (2 
minutes  in  the  spot  test  and  one-quarter  to  one-half  minute  in  the 
filter  test).  As  a  rule  the  spot  and  filter  titrations  only  differ  from 
0.2  to  0.6  c.c.  of  copper  solution,  when  from  8  to  12  c.c.  of  the  above 
standard  have  been  used,  and  the  finishing  point  may  be  determined 
to  0.2  c.c. 

Special  Modification  of  Knublauch's  Method. — In  some  cases,  in- 
stead of  the  blue  color  disappearing  sharply  at  a  certain  stage,  it  is 
found  that  the  solution,  in  the  filter  test,  remains  persistently  green 
or  greenish-yellow,  so  that  the  filter  test  registers  much  higher  than 
the  spot  test ;  moreover,  in  such  cases  the  spot  test  with  ferric  chlo- 
ride registers  higher  than  the  spot  test  on  filter  paper  alone.  This 
is  stated  to  be  due  to  the  presence  of  other  substances  containing 
iron  and  cyanogen  in  varying  quantities,  and  also  containing  sul- 
phur. These  substances  appear  to  undergo  gradual  decomposition, 
yielding  ferrocyanides,  and  they  are  also  decomposed  by  potash 
and  precipitated  by  ferric  chloride. 

The  modified  method  proposed  in  such  cases  is  as  follows :  Add 
to  200  or  300  c.c.  of  the  solution,  slightly  more  copper  sulphate 
solution  than  is  required  by  the  spot  titration;  filter,  pour  the  fil- 
trate into  hot  ferric  chloride  (to  precipitate  any  residual  ferro- 
cyanogen  not  indicated  by  the  copper  titration) ;  decompose  and  ti- 
trate the  Prussian  blue  formed,  and,  if  it  is  thought  desirable,  the 
filtrate  from  this  precipitate  is  treated  in  the  same  manner.  The 
numbers  obtained  in  this  way  are  added  as  corrections  to  the  num- 
bers obtained  in  the  usual  manner,  and  it  is  then  found  that  the 
spot  and  filter  titrations  approximate  more  and  more,  the  latter 
becoming  less,  the  former  increasing.  It  therefore  seems  that  in 
these  cases  the  spot  test  shows  too  little  and  the  filter  test  too 
much. 


82  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

METHOD  No.  3. 

Estimation  of  Ferrocyanides  by  Conversion  into  Ferricyanides,  and 

Titration  with  Copper  Nitrate  with  Ferrous  Sulphate 

as  External  Indicator. 

F.  Hurter  (Chemical  News,  Vol.  XXXIX.,  p.  25)  describes  the 
following  method  for  determining  small  quantities  of  ferrocyanides 
in  presence  of  cyanates,  thiocyanates,  cyanides,  sulphides  and  thio- 
sulphates:  100  c.c.  of  the  solution  are  boiled  with  solution  of 
bleaching  powder,  in  quantity  sufficient  to  convert  all  sulphides  and 
thiosulphates  into  sulphates,  and  the  ferrocyanide  into  ferricyanide. 
The  liquid  is  then  acidified  and  freed,  as  far  as  possible,  from  the 
excess  of  chlorine  by  warming  and  agitating  it.  It  is  then  titrated 
with  N/20  solution  of  cupric  nitrate,  prepared  by  dissolving  3.17 
grams  of  metallic  copper  in  as  little  nitric  acid  as  possible,  and  dilut- 
ing to  one  liter.  On  adding  this  solution  to  the  acidulated  liquid 
containing  ferricyanide,  a  yellow  precipitate  of  cupric  ferricyanide 
is  formed.*  Drops  of  the  thoroughly  mixed  liquid  are  taken  up 
with  a  glass  rod  and  added  to  drops  of  a  1  per  cent  solution  of  crys- 
tallized ferrous  sulphate  on  a  porcelain  plate.  As  long  as  insufficient 
copper  solution  has  been  added  to  combine  with  the  whole  of  the  fer- 
ricyanide present,  the  deep  blue  ferrous  ferricyanide  is  formed  on 
the  porcelain.  When  the  liquid  no  longer  contains  soluble  ferricya- 
nide the  indicator  acts  on  the  copper  precipitate,  and  reduces  it  to  the 
characteristic  chocolate-colored  cupric  ferrocyanide.  Hence  the  end 
of  the  reaction  is  indicated  by  a  brown  color  being  produced  on  the 
porcelain  instead  of  the  blue  first  obtained.  Each  c.c.  of  the  copper 
solution  added  before  this  result  is  obtained  represents  0.01013 
gram  of  Na4FeCy6  in  the  liquid.  The  method  is  not  suitable  for 
the  determination  of  large  quantities  of  ferrocyanides,  as  the  color 
of  the  copper  precipitate  obscures  the  blue  color,  and  the  precipitate 
is  not  always  of  definite  composition. 

It  is  evident  that  this  process  is  also  adapted  for  the  direct  esti- 
mation of  small  quantities  of  ferrocyanide. 

A  practically  identical  method  is  described  by  Sutton  (Volum. 
Anal.,  8th  ed.,  p.  226). 

Method  No.  3  (a).— The  following  modification  of  Hurter's 
»Cu,(FeCy.),. 


CHEMISTEY    OF    CYANIDE    SOLUTIONS.  83 

method  appears  to  be  a  simple  and  accurate  method  of  estimating 
ferrocyanides  in  presence  of  zinc. 

1.  Heat  the  solution  to  about  80°  C.,  make  strongly  alkaline  with 
NaOH,  add  Na2S  in  slight  excess,  agitate,  settle,  filter  and  wash 
well  with  hot  water.     The  filtrate  is  practically  free  from  zinc. 

2.  Cool  the  solution,  acidulate  with   H2S04,  and  add  N/10 
KMn04  till  H2S  and  other  reducing  agents  are  completely  oxidized, 
and  a  faint  reddish  tint  is  permanent. 

3.  Titrate  with  standard  copper  sulphate  solution,  using  ferrous 
ammonium  sulphate  as  indicator,  in  spots,  on  a  white  plate.     The 
end  point  is  very  sharp  and  definite. 


GROUP  (D). — ESTIMATION  OF  FERBOCYANIDE  BY  KEACTIONS  WITH 
ZINC  SALTS. 

METHOD  No.  1. 
Titration  with  Zinc  Sulphate,  Using  Ferric  Chloride  as  Indicator, 

A  method  based  on  this  principle  is  described  by  J.  Miiller 
(Wagner's  Jahresbericht,  1861,  p.  238),  and  K.  Zulkowski  (Jdhres- 
lericht,  1883,  p.  491).  According  to  the  latter,  constant  results  can 
only  be  obtained  with  hot  solutions.  A  suitable  standard  is  ob- 
tained by  dissolving  111  grams  of  the  double  salt  (ZnSCVKjSCV 
6H20)  in  water  and  diluting  to  one  liter. 

Ten  c.c.  of  this  zinc  solution  are  measured  out,  mixed  with  5  c.c. 
of  dilute  sulphuric  acid  (1:5)  and  20  c.c.  of  water,  and  the  whole 
is  heated  to  boiling.  The  solution  to  be  tested  is  run  from  a  burette 
into  the  hot  liquid  until,  when  a  drop  of  the  mixture  is  put  on  a 
piece  of  filter  paper  and  a  drop  of  very  dilute  ferric  chloride  solution 
put  on  the  zone  of  paper  soaked  in  the  solution,  it  just  produces 
a  blue  color.  The  precipitate  remains  localized  in  the  place  where 
the  drop  was  put  down.  The  zinc  solution  should  be  standardized 
on  a  model  solution  prepared  from  pure  ferrocyanide,  containing 
approximately  the  same  percentage  as  the  solution  to  be  tested.  Ac- 
cording to  Thorpe  (Diet,  of  Applied  Chemistry,  Art.  'Cyanide') 
the  method  gives  only  moderately  exact  results. 

In  applying  the  above  process  E.  Gasch  (Abstract  in  Journal 
Chem.  Soc.,  1890,  p.  834)  employs  a  1  per  cent  solution  of  uranium 
acetate,  instead  of  ferric  chloride.  With  this  indicator,  ferrocyanides 


84  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

give  a  brown  coloration.  Gasch  also  uses  a  standard  2  per  cent  so- 
lution of  potassium  ferrocyanide,  against  which  he  titrates  the  zinc 
solution,  instead  of  using  a  standard  solution  of  zinc  sulphate,  as  in 
Zulkowsk/s  original  process. 

When  the  ferrocyanide  is  only  present  in  very  small  amount  it  is 
preferably  precipitated  as  Prussian  blue,  filtered  and  dissolved  in 
caustic  alkali  solution,  after  which  it  is  titrated  as  described  above. 


METHOD  No.  2. 

Estimation  of  Ferrocyanide  by  Titration  of  Alkali  Generated  on 
Addition  of  Zinc  Carbonate. 

The  following  method  is  described  by  E.  Zaloziecki  (Zeitschrift  f. 
Anal.  Chem.,  Vol.  XXX.,  p.  484;  see  abstract  in  Chemical  News, 
1891,  p.  207).  It  is  based  on  the  fact  that  ferrocyanide  of  po- 
tassium or  sodium  may  be  completely  precipitated,  in  the  form  of 
double  ferrocyanide  of  zinc  and  alkali  metal,  by  the  addition  of 
zinc  carbonate,  and  subsequent  passage  of  a  current  of  carbonic 
acid  gas.  The  double  ferrocyanide  is  then  transformed  into  the 
corresponding  sodium  or  potassium  carbonate,  and  the  quantity  of 
ferrocyanide  originally  present  found  by  titrating  the  alkaline  car- 
bonate formed.  According  to  Zaloziecki,  3  molecules  of  K4FeCy6 
yield  on  decomposition  2  molecules  of  Zn2FeCy6,  whilst  1  molecule 
of  K4FeCye  remains  undecomposed.  The  double  salt,  therefore, 
corresponds  to  the  formula, 

2Zn2FeCy6  +  K4FeCy6, 

its  decomposition  by  zinc  carbonate  being  represented  by  the  follow- 
ing equation : 
.       3K4FeCy6  +  4ZnC03  =  (2Zn2FeCye)  -K4FeCy6  +  4K2C03. 

With  potassium  ferrocyanide  the  reaction  takes  place  hot  or  cold. 
With  sodium  ferrocyanide  the  above  reaction  only  takes  place  in  hot 
solutions,  the  reaction  in  the  cold  giving  a  double  salt  poorer  in  zinc. 
For  this  reason  it  is  necessary  always  to  operate  with  hot  solutions. 
Four  parts  of  carbonate  found  should  correspond  to  three  parts  of 
ferrocyanide  in  the  original  substances. 

When  cyanides  or  other  substances  alkaline  to  the  indicator  used 
are  also  present,  they  must,  of  course,  be  removed  or  allowed  for. 
This  reaction  between  ferrocyanides  and  zinc  carbonate  forms  the 
basis  of  the  methods  recently  described  (Oct.,  1901)  by  L.  M. 


CHEMISTRY    OF    CYANIDE    SOLUTION'S.  85 

Green,  in  a  paper  laid  before  the  Institute  of  Mining  and  Metal- 
lurgy for  estimating  zinc,  ferrocyanides  and  alkali  in  cyanide  solu- 
tions containing  zinc.  The  method  will  be  more  conveniently  dis- 
cussed in  connection  with  the  determination  of  zinc. 


GROUP   (E). — ESTIMATION  OF  FERROCYANIDE  BY  DETERMINING 

THE  IRON  CONTENTS  AFTER  COMPLETE  DECOMPOSITION 

OF  FERROCYANOGEN. 

METHOD  No.  1. 
Treatment  by  Evaporation  with  Mineral  Acidi. 

According  to  W.  J.  Sharwood  (Engineering  and  Mining  Journal., 
1898,  p.  216),  the  methods  for  estimation  of  ferrocyanides  and 
thiocyanates  based  upon  oxidation  by  permanganate  were  found  to 
be  totally  unreliable  when  tested  experimentally  upon  solutions  con- 
taining known  quantities  of  the  substances  accompanying  them  in 
cyanide  solutions. 

He  therefore  takes  100  c.c.  of  the  solution,  evaporates  twice  with 
nitric  acid,  redissolves  the  residue  in  dilute  sulphuric  acid,  and  pre- 
cipitates the  iron  by  excess  of  ammonia.  The  precipitate  is  then  at 
once  redissolved  in  hydrochloric  acid,  and  the  iron  estimated  colori- 
metrically  as  thiocyanate,  unless  the  quantity  is  sufficient  to  allow  of 
reduction  by  zinc  and  titration  with  permanganate, 
Fe  X  7.541  =  K4FeCy6-3H20. 

The  method  advocated  by  Moldenhauer  and  Leybold  (see  Allen, 
Comm.  Org.  Anal.,  Vol.  III.,  pt.  3,  p.  467)  is  practically  identical 
with  the  above. 

METHOD  No.  2. 
Decomposition  of  Ferrocyanide  after  Precipitation  as  Prussian  Blue. 

The  following  method,  due  to  W.  Leybold  (Journ.  f.  Gasbeleuch- 
tung,  XXXIII.,  pp.  427-428),*  is  intended  specially  for  the  estima- 
tion of  cyanogen  in  coal  gas,  this  impurity  being  removed  and  con- 
verted into  ferrocyanide  by  passing  the  gas  through  vessels  containing 

»  See  also  Journal  Soc.  Chem.  Ind.,  Vol.  IX.,  pp.  923-979,  1890. 


86  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

ferrous  hydrate.    The  method,  so  far  as  concerns  the  estimation  of 
f errocyanide,  is  as  follows : 

The  filtered  liquid  is  acidified  with  HC1  and  ferric  chloride 
(1:10)  added  in  excess.  The  precipitate  of  Prussian  blue  is 
filtered  off  and  washed  till  the  washings  are  colorless,  put  in  a  beaker 
with  the  filter  paper  and  a  little  caustic  soda  added.  After  decom- 
position, the  ferric  oxide  is  filtered  off  and  washed  till  free  from 
ferrocyanogen.  The  filtrate  is  then  evaporated  in  a  platinum  basin 
to  about  30  c.c.  and  strongly  acidified  with  sulphuric  acid  (1 : 10), 
evaporated  to  dryness  and  ignited.  The  residue  is  then  extracted 
with  100  c.c.  of  dilute  sulphuric  acid,  washed  with  50  c.c.  of  water 
and  poured  into  a  250  c.c.  flask.  One  c.c.  of  copper  sulphate  solu- 
tion (1:  10)  and  10  grams  of  pure  zinc  are  added,  and  the  whole 
allowed  to  stand  till  completely  reduced.  The  iron  is  then  deter- 
mined by  permanganate  in  the  usual  way. 

COMPAEATIVE  TESTS  WITH  DIFFERENT  METHODS  OF  ESTIMATING 
FERROCYANIDE. 

A  model  solution  was  prepared  of  the  following  theoretical  con> 
position : 

Free  cyanide,  0.2%.  Thiocyanide,  KCyS,  0.04%. 

Cyanide  as  K2ZnCy4,  0.2%.  Sodium  bicarbonate,  0.39%. 

Ferrocyanide,  K4FeCy6  3H20,  0.2%.   Sodium  carbonate^  0.067o. 
Ammonium  chloride,  0.1%. 

This  was  tested  for  f  errocyanide : 

(A)  By  evaporation  with  acids  and  determination  of  total  iron 
(Group  E,  No.  1). 

(B)  By  Erlenmeyer's  method  (Group  A,  No.  2). 
The  results  were : 

Method  A.  Method  B. 
0.203%  0.203% 

0.211%;  0.219% 

0.207%  0.194% 

Mean,  0.29J%  0.205%, 

Tests  made  by  Bettel's  method  (running  liquid  to  be  tested  into 
acidulated  permanganate)  were  indefinite  and  totally  erroneous. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  87 

SECTION   C. 
ESTIMATION  OF  THIOCYANATES  (SULPHOCYANIDES). 

The  methods  proposed  may  be  classified  as  follows: 

(A)  Methods  depending  on  oxidation  of  thiocyanate  by  perman- 
ganate. 

(B)  Methods  depending  on  oxidation  of  thiocyanate  by  iodine  in 
neutral  solution. 

(C)  Colorimetric  method,  using  ferric  salts. 

(D)  Precipitation  with  copper  salts  in  presence  of  a  reducing 
agent. 

GROUP  (A) . — ESTIMATION  OF  THIOCYANATES  BY  OXIDATION  WITH 
PERMANGANATE. 

This  method  has  been  already  alluded  to  in  discussing  the  re- 
actions of  permanganate  with  ferrocyanides,  and  in  connection 
with  Bettel's  method  (B.a.3)  for  estimating  the  latter.  It  de- 
pends on  the  reaction : 

6KMn04  +  12H2S04  -f  5KCyS  = 
11KHS04  +  6MnS04  +  5HCy  +  4H20. 

It  will  be  observed  that  the  reducing  power  of  thiocyanates  is 
much  greater,  weight  for  weight,  than  that  of  ferrocyanides,  1  c.c. 
N/100  KMn04  =  0.0001618  gram  KCyS, 

=  0.003684    gram  K4FeCy6. 

Hence  it  is  preferable  to  use  a  N/10  solution  of  permanganate  in 
this  case. 

When  ferrocyanides  are  also  present  they  may  be  separated  by 
precipitation  as  Prussian  blue  as  already  described,  but  the  pre- 
cipitate must  be  very  thoroughly  washed.  Other  forms  of  oxidiz- 
able  organic  matter  are  previously  removed  by  shaking  with  lime 
and  filtering.  (See  above.) 

GROUP  (B). — ESTIMATION  OF  THIOCYANATES  BY  OXIDATION  WITH 
IODINE.* 

Thiocyanates  (sulphocyanides)  react  with  iodine  in  presence  of 
an  alkaline  bicarbonate,  as  follows: 


•Rupp  and   Schied  :   Berichte  35    [12],   2191-2195;   see  also  Journal  Soc. 
m.  Ind.,  " 


Chem.  Ind.,  1902,  Oct.  31. 


88  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

KCNS  +  KHC03  +  81  +  3H20  = 

KHS04  +  6HI  +  C02  +  KI  +  ION. 

In  the  first  account  of  the  method,  the  following  description  is 
given:  "A  known  quantity  of  thiocyanate  solution  is  allowed  to 
stand  with  excess  of  N/IO  iodine  and  about  1  gram  of  sodium  bi- 
carbonate in  a  stoppered  bottle  for  half  an  hour  in  the  dark,  and  the 
excess  of  iodine  titrated  with  sodium  thiosulphate." 

A.  Thiel  (Berichte,  35  [15],  2766),  gives  a  modification  of  this 
as  follows:  "Ten  c.c.  of  the  thiocyanate  solution  and  2  grams 
of  sodium  bicarbonate,  with  sufficient  water  to  effect  com- 
plete solution,  are  placed  in  a  loosely-stoppered  flask  with  50  c.c.  of 
N/5  iodine  solution,  and  allowed  to  stand  at  room  temperature  for 
four  hours.  N/2  hydrochloric  acid  is  then  added,  and  the  excess  of 
iodine  titrated  with  N/10  thiosulphate  solution." 

Kupp  and  Schied  state  that  the  presence  of  cyanogen  iodide 
prevents  the  use  of  starch  as  an  indicator,  and  on  account  of  the 
yellow  color  produced  by  the  same  compound  with  KI,  it  is  ad- 
visable to  work  with  such  quantities  that  not  more  than  20  c.c.  of 
iodine  solution  are  required.  The  end  of  the  reaction  is  shown  by 
the  disappearance  of  the  yellow  color.  Shaking  the  bottle  should 
be  avoided  in  order  to  prevent  the  evolution  of  carbonic  acid.  The 
method  gives  good  results  in  presence  of  chlorides. 

Procedure  in  Presence  of  Cyanides. — In  presence  of  cyanides,  boil- 
ing with  about  £  gram  of  tartaric  acid  for  15  minutes  in  an  open 
flask  is  sufficient  to  get  rid  of  all  hydrocyanic  acid  and  the  thio- 
cyanate may  then  be  estimated  as  above.  In  the  same  way  a  mix- 
ture of  thiocyanate  with  chloride  and  cyanide  may  be  estimated  thus : 
(a)  10  c.c.  of  the  solution  are  precipitated  with  20  c.c.  of  AgN03, 
acidulated  with  HN"03,  the  solution  made  up  to  100  c.c.  and  fil- 
tered, the  excess  of  silver  being  estimated  in  the  filtrate  with 
N/10KCNS.  (Volhard's  method.)  Eesult  (a).  Fifty  c.c.  of  the 
original  solution  are  boiled  for  15  minutes  with  1  gram  of  tartaric 
acid  to  decompose  the  cyanide,  and  the  solution  made  up  to  100  c.c. 
In  20  c.c.  of  this  diluted  solution  the  chloride  and  thiocyanate  are 
determined  by  Volhard's  method.  Result  (6).  And  in  a  further 
10  c.c.  the  thiocyanate  (alone)  is  estimated  with  iodine  as  above. 
Result  (c). 

(a)  =  KC1  +  KCNS  +  KCN. 

(&)=KCI+ 

(c)  =  KCNS. 


V 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  89 

I  may  add  that  I  have  tested  this  method,  with  satisfactory  re- 
sults. When  zinc  is  present,  a  precipitate  occurs  on  boiling  with 
tartaric  acid,  but  this  may  be  filtered  off  and  the  thiocyanate  esti- 
mated in  the  filtrate  without  difficulty. 

GROUP  (C). — ESTIMATION  OF  THIOCYANATES  BY  COLORIMETBIC 
METHOD. 

As  is  well  known,  ferric  salts  give  a  very  delicate  reaction  for 
thiocyanates ;  a  very  small  trace  is  sufficient  to  produce  in  neutral 
or  slightly  acid  solutions  an  intense  and  characteristic  blood-red 
tinge. 

The  test  may  be  made  as  follows  (W.  J.  Sharwood,  Engineering 
and  Mining  Journal,  1898,  p.  216) :  10  or  20  c.c.  of  the  solution 
to  be  tested  are  acidulated  with  hydrochloric  acid,  ferric  chloride 
added,  and  the  color  compared  with  that  produced  by  standard 
thiocyanate  under  the  same  conditions.  When  ferrocyanides  are 
also  present  the  precipitate  formed  must  be  filtered  off  before  the 
tint  can  be  compared,  and  the  precipitate  well  washed. 

GROUP  (D). — ESTIMATION  OF  THIOCYANATES  BY  PRECIPITATION 
AS  CUPROUS  SALT,  USING  FERROCYANIDE  AS  INDICATOR.* 

This  method  depends  upon  the  fact  that  when  a  solution  of  a 
cupric  salt  is  added  to  a  solution  of  a  thiocyanate  in  presence  of  a 
reducing  agent,  such  as  sodium  bisulphite,  the  insoluble  cuprous 
salt  of  thiocyanic  acid  is  precipitated,  the  end  of  the  reaction  being 
ascertained  by  a  drop  of  the  solution  in  the  flask  giving  a  brown 
coloration  when  brought  in  contact  with  a  drop  of  ferrocyanide. 

The  following  reaction  takes  place : 

2CuS04  +  2KCN"S  +  Na^O,  +;  H20  = 

Cu2S(SCN)2  +  K2S04  -f  2NaHS04. 

The  solutions  required  are: 

(1)  A  standard  solution  of  cupric  sulphate  containing  6.2375 
grams  per  liter,  1  c.c.  of  which  is  equivalent  to  0.00145  gram  SCN". 

(2)  A  solution  of  sodium  bisulphite  of  sp.  gr.  1.3. 

(3)  A  solution  of  potassium  ferrocyanide  1 :  20. 

The  liquid  to  be  tested  must  be  boiled  after  addition  of  the  bi- 
sulphite, about  3  c.c.  of  the  latter  to  25  c.c.  of  the  liquid  to  be  ex- 
amined. A  measured  volume  of  the  copper  solution  is  then  run  in, 

*  Barnes  and  Liddle,  Journal  Soc.  Cbem.  Ind.,  II.,  p.  122 ;  see  also  Button, 
Volum.  Anal.,  8th  ed.,  p.  228. 


90  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

well  shaken,  and  the  precipitate  allowed  to  settle  for  about  a  minute. 
A  drop  taken  out  on  a  glass  rod  and  brought  in  contact  with  a 
drop  of  ferrocyanide  on  a  white  plate  gives  an  immediate  brown 
coloration  as  soon  as  an  excess  of  copper  is  present.  The  change 
must  be  immediate,  as  a  brown  color  develops  on  standing,  even 
when  the  reaction  is  not  complete.  The  test  should  be  repeated  after 
a  preliminary  approximation  has  been  obtained,  finally  adding  the 
copper  solution  drop  by  drop  when  near  the  finishing  point. 


SECTION  D. 
ESTIMATION  or  SULPHIDES. 

Alkaline  sulphides  are  rarely  found  in  working  cyanide  solutions, 
although  a  small  percentage  frequently  occurs  in  samples  of  com- 
mercial cyanide.  Where  the  zinc  precipitation  process  is  used,  it 
is  probable  that  the  zinc  double  cyanide  in  the  solution  precipitates 
as  ZnS  any  small  quantity  of  sulphide  which  may  be  present. 

When  soluble  sulphides  occur  in  the  solution  or  in  commercial 
cyanide,  they  may  be  estimated  by  one  or  other  of  the  following 
methods : 

1.  Gravimetrically,  in  several  ways ;  for  example,  by  precipitation 
with  lead  salts,  and  subsequent  oxidation  to  sulphate. 

2.  By  precipitation  as  lead  sulphide,  and  conversion  into  sul- 
phocyanide  by  means  of  pure  cyanide. 

3.  Colorimetrically,  by  means  of  sodium  nitroprusside. 

4.  By  means  of  double  silver  cyanide. 

Numerous  other  methods  might  be  used  with  suitable  modifica- 
tions to  make  them  applicable  to  complex  cyanide  solutions.  (See 
Fresenius,  Quant.  Anal.,  7th  ed.,  Vol.  I.,  pp.  380-390.) 

METHOD  No.  1. 
Gravimetric  Estimation  of  Sulphides  by  Means  of  Lead  Salts. 

An  excess  of  carbonate  of  lead,  or  an  alkaline  lead  salt  such  as  a 
solution  of  oxide  of  lead  (plumbate),  or  tartrate  of  lead  in  excess  of 
caustic  alkali  is  added  to  the  cyanide  solution,  and  the  precipitate 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  91 

of  lead  sulphide  collected  on  a  filter  and  washed.  The  precipitate  is 
then  oxidized  by  means  of  chlorine  or  bromine  in  presence  of  an 
excess  of  alkaline  hydrate,  filtered,  and  the  solution  acidified.  The 
excess  of  chlorine  or  bromine  is  then  boiled  off,  the  sulphuric  acid 
precipitated  by  barium  chloride  and  determined  in  the  usual  way. 
The  method  is  stated  to  be  accurate  but  tedious. 


METHOD  No.  2. 

Precipitation  by  Means  of  Lead  Salts  and  Conversion  into  Thio- 
cyanate. 

This  method  is  described  by  Feldtmann  and  Bettel  (Proceedings 
Chem.  and  Met.  Soc.  South  Africa,  Vol.  I.,  pp.  267-273).  It  is 
based  on  the  fact  that  lead  sulphide  is  decomposed  by  alkaline  cyan- 
ides on  exposure  to  bright  sunlight,  or  in  presence  of. hydrogen 
peroxide,  with  conversion  of  the  sulphur  into  thiocyanate.  The 
latter  may  then  be  estimated  by  one  of  the  methods  described  above 
(e.g.,  by  titration  of  the  acidulated  solution  with  N/100  perman- 
ganate), and  the  amount  of  sulphide  found  by  calculation. 

The  method  is  as  follows : 

The  solution  is  agitated  with  a  slight  excess  of  precipitated  lead 
carbonate  and  filtered.  The  precipitate,  consisting  of  lead  carbon- 
ate and  sulphide,  is  transferred  to  a  flask  and  covered  with  a  few  c.c. 
of  a  solution  of  potassic  or  sodic  cyanide  free  from  sulphides,  sul- 
phocyanides  or  f errocyanides.  This  is  best  prepared  from  pure  potas- 
sic or  sodic  hydrate  and  pure  hydrocyanic  acid.  A  slight  excess  of 
hydrogen  peroxide  is  then  added,  about  3  or  4  times  as  much  as  is 
necessary  to  whiten  the  precipitate,  and  allowed  to  act  for  from  10 
to  15  minutes.  A  small  quantity  (say  half  a  gram)  of  manganese 
dioxide  is  added,  and  the  mixture  agitated  for  about  two  minutes  to 
destroy  the  excess  of  hydrogen  peroxide. 

H202  +  2Mn02  =  H20  -f  Mn203  -f  02. 
Mn203  -f  H202  =  H20  -f  2Mn02. 

The  solution  is  then  filtered  off,  acidified  with  sulphuric  acid,  and 
titrated  with  N/100  permanganate. 

1  c.c.  N/100  KMn04  =  0.000053  gram  S. 

=  0.000182  gram  K,S. 
=  0.0001618  OTam  KCNS. 


92  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

The  same  conversion  may  be  effected  without  the  use  of  hydrogen 
peroxide  by  merely  exposing  the  mixture  of  lead  sulphide  and  pure 
cyanide  to  bright  sunlight  for  several  hours. 


METHOD  No.  3. 

Colorimetric    Estimation   of    Sulphides    by    Means    of    Sodium 
Nitroprusside. 

This  method,  devised  by  Dr.  J.  Loevy  of  Johannesburg,  is  very 
rapid  and  simple.  The  solutions  required  are: 

(a)  Standard  sodium  sulphide:  40  grams  Na2S  and  0.2  gram 
NaOH  dissolved  to  a  liter.  1  c.c.  of  this  solution  =  0.0053  gram. 

(6)  Standard  zinc  sulphate:  44.15  grams  ZnS04-7H20  per  liter. 
1  c.c.  =  0.01  gram  Zn. 

(c)  Sodium  nitroprusside :  5  grams  of  the  salt  dissolved  in 
100  c.c.  water,  to  which  4  to  6  drops  of  5  per  cent  cyanide  are 
added. 

For  the  estimation  of  sulphides  in  samples  of  commercial  cyanide 
the  method  is  carried  out  as  follows: 

A.  Ten  grams  of  the  cyanide  to  be  examined  are  dissolved  in 
water  and  made  up  to  500  c.c.    Of  this  solution,,  100  c.c.  are  placed 
in  a  cylinder. 

B.  Ten  grams  of  cyanide  free  from  sulphides  is  also  dissolved 
to  500  c.c.  and  100  c.c.  placed  in  another  similar  cylinder. 

One  c.c.  of  the  nitroprusside  solution  is  then  added  to  each 
cylinder,  which  gives  an  intense  violet  color  when  sulphides  are 
present.  The  standard  sulphide  solution  (a),  diluted  in  the  pro- 
portion 1 : 10,  is  now  added  drop  by  drop  to  the  cylinder  B  (contain- 
ing pure  cyanide)  until  the  tint  is  the  same  in  each. 

The  sodium  sulphide  solution  is  standardized  by  means  of  the 
zinc  solution  (&),  using  filter  paper  dipped  first  in  ferric  chloride, 
then  in  very  dilute  ammonia  as  an  indicator. 

[I  find  that  this  may  be  done  much  more  easily,  and  probably 
more  accurately,  by  making  use  of  the  method  (No.  4)  described 
below.] 

1  c.c.  zinc  solution  =  0.00499  gram  sulphur. 

Preparation  of  Sodium  Nitroprusside. — Concentrated  nitric  acid 
is  diluted  with  its  own  volume  of  water.  This  diluted  acid  is  mixed 
with  powdered  potassium  ferrocyanide  in  the  proportion: 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  93 

2  parts  K4FeCye-3H20, 

5  parts  diluted  HN03. 

The  salt  dissolves  to  a  coffee-colored  liquid  evolving  C02,  N,  CN 
and  HCN.  It  is  warmed  on  a  water  bath  until  the  liquid  gives  a 
dark  green  or  slate  colored  precipitate  instead  of  a  blue  precipitate 
with  ferrous  sulphate.  It  is  then  cooled,  neutralized  with  sodium 
carbonate  and  filtered. 

METHOD  No.  4. 

Estimation  of  Sulphides  by  Adding  Double  Cyanide  of  Silver  and 
Titrating  the  Free  Cyanide  Liberated. 

On  adding  an  excess  of  a  solution  of  the  double  cyanide  of  silver 
to  a  liquid  containing  alkaline  sulphides,  free  cyanide  is  produced 
in  proportion  to  the  amount  of  sulphide  present,  thus : 
2KAgCy2  +  K2S  =  Ag2S  +  4KCy. 

After  filtering  off  the  precipitated  sulphide  of  silver,  the  cyanide 
is  estimated  by  adding  potassium  iodide  and  titrating  in  the  ordinary 
way  with  silver  nitrate. 

Any  cyanide  originally  present  in  the  liquid  must  be  separately 
determined  after  treatment  with  lead  carbonate,  and  the  amount 
deducted  from  that  previously  found. 

For  rapid  approximate  results,  the  solution,  after  addition  of  the 
silver  double  cyanide  and  thorough  agitation,  may  be  made  up  to  a 
definite  volume,  and  an  aliquot  part  filtered  off  for  titration.  The 
silver  sulphide  generally  settles  rapidly  and  is  easily  filtered;  a  little 
lime  may,  however,  be  added  with  advantage,  in  some  cases. 

The  results  obtained  in  numerous  tests  with  this  method  were 
strictly  proportional  to  the  amount  of  sulphide  present. 

The  silver  double  cyanide  is  prepared  by  adding  silver  nitrate  to  a 
solution  of,  say,  0.5  per  cent  KCy  until  a  slight  permanent  tur- 
bidity is  formed,  allowing  to  stand  for  some  time,  and  filtering. 


SECTION   E. 
OTHER  KEDUCINQ  AGENTS. 

Estimation  of  Nitrites. — The  presence  of  nitrites  may  be  de- 
tected by  the  liberation  of  iodine  when  potassium  iodide  is  added 


94  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

to  a  solution  slightly  acidulated  with  sulphuric  acid.  Cyanides, 
ferrocyanides  and  thiocyanates  do  not  interfere,  but  ferricyanides 
must  be  absent,  as  they  give  the  same  reaction.  The  haloid  com- 
pounds of  cyanogen  also  decompose  potassium  iodide  in  the  same 
way. 

Nitrites  also  reduce  permanganate,  and  would  be  determined 
along  with  thiocyanates,  etc.,  in  the  processes  described  above. 

They  may  be  estimated  in  cyanide  solutions  by  the  iodometric 
method  of  Dunstan  and  Dymond  (Phar.  Journ.  [3]  XIV.,  p.  741, 
and  Sutton,  Volum.  Anal.,  8th  ed.,  p.  292),  which  depends  on  the 
reaction  with  potassium  iodide  just  described.  The  operation  must 
be  conducted  in  absence  of  air,  and  the  solution  of  potassium  iodide 
and  sulphuric  acid  must  be  boiled  to  expel  air  and  any  traces  of 
free  iodine  before  admitting  the  cyanide  solution  in  which  the  nitrite 
is  to  be  estimated.  When  the  reaction  is  complete,  the  liberated 
iodine  is  determined  in  the  usual  way  by  standard  thiosulphate  with 
or  without  starch  indicator. 

The  potassium  iodide  and  sulphuric  acid  (10  per  cent  solution) 
are  first  boiled  in  a  flask  provided  with  a  funnel  connected  by  means 
of  a  piece  of  rubber  tubing  and  a  screw  clamp,  so  that  it  may  be 
closed  air-tight  as  soon  as  all  air  has  been  expelled  and  steam  is  issu- 
ing from  the  flask.  The  liquid  to  be  examined  is  then  introduced 
through  the  funnel  by  opening  the  clamp  cautiously,  and  the  funnel 
is  then  washed  with  water  free  from  air.  The  thiosulphate  solu- 
tion may  be  admitted  through  the  funnel  in  the  same  way. 

The  reactions  occurring  are : 

(a)  H2S04  +  2KI  =  K2S04  +  2HI. 

(6)  H2S04  +  2KN02  =  K2S04  +  2HN02. 

(c)  2HI  +  2HN02  =  2H20  +  2NO  +  I2. 


CHEMISTBY    OF    CYANIDE    SOLUTIONS.  95 


CLASS  IV. 
AUXILIARY  AGENTS. 

General  Remarks. — We  shall  include  under  this  heading  any  sub- 
stance which  hastens  the  rate  of  solution  of  the  precious  metals  by 
cyanide,  or  which  causes  the  extraction  of  these  metals  from  com- 
pounds which  would  not  otherwise  be  attacked. 

From  what  has  already  been  said,  it  is  evident  that  oxygen,  or  any 
substance  capable  of  supplying  oxygen  directly  or  indirectly  to  the 
solution,  is  in  this  sense,  under  ordinary  circumstances,  an  auxiliary 
agent.  There  are  also  some  other  substances  which  do  not  contain 
oxygen,  and  which  do  not,  at  all  events,  act  directly  as  carriers  of 
oxygen,  which  nevertheless  exert  a  marked  influence  in  increasing 
the  rate  of  solution. 

We  shall  here  consider : 

A.  Oxygen.  C.    Peroxides. 

B.  Active  Haloids.  D.    Ferricyanides. 


SECTION  A. 
ESTIMATION  OF  OXYGEN. 

A  correct  determination  of  the  amount  of  free  oxygen  or  its 
equivalent  would  be  of  the  greatest  value  in  determining  the  effi- 
ciency of  a  cyanide  solution.  Unfortunately  no  method  has  so  far 
been  suggested  which  does  not  involve  more  or  less  complicated 
operations  and  delicate  manipulations.  Two  methods  have  been 
proposed  for  determining  the  amount  of  free  oxygen  in  cyanide 
solutions,  both  of  them  based  upon  well-known  processes  for  the 
determination  of  dissolved  oxygen  in  water.  These  are: 

1.  The  gasometric    method,  based    on    that    of    Lunge    and 
Schmidt  (Zeitschrift  f.  Anal.  Chem.,  Vol.  XXV.,  p.  309). 

2.  The  iodometric  method,  based  on  Thresh's  process. 


96  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


METHOD  No.  1. 

Estimation  of  Oxygen  by  Process  Based  on  the  Gasometric  Method 
of  Lunge  and  Schmidt. 

An  account  of  this  process  is  given  by  Bettel  (Proceedings  Chem. 
and  Met.  Soc.  South  Africa,  Vol.  I.,  p.  276),  its  application  to 
cyanide  solutions  being  based  on  the  assumption  that  the  dissolved 
oxygen  is  expelled  without  appreciably  attacking  the  cyanide  or  the 
impurities  contained  in  the  solution  when  the  latter  is  boiled  at 
80°  C.  under  reduced  pressure  for  about  15  minutes.  This  does  not 
apply  to  sulphides,  which  must  be  removed  before  making  the  test. 

The  process  is  identical  with  that  described  in  Sutton's  Volumetric 
Analysis  (8th  ed.,  p.  613),  where  full  details  are  given  as  to  the 
apparatus  and  manipulations  required.  It  involves  the  use  of  a 
nitrometer,  consisting  of  a  graduated  tube  or  burette  connected  at 
the  upper  end  by  means  of  a  two-way  cock  with  a  small  funnel  and 
with  a  side  tube,  which  can  be  joined  by  rubber  tubing  with  the 
flask  containing  the  solution  to  be  examined. 

When  the  method  is  applied  to  cyanide  solutions,  the  temperature 
and  specific  gravity  of  the  liquid  must  be  taken  for  the  purposes  of 
calculation. 

When  sulphides  are  present,  carbonate  of  lead  is  added,  suspended 
in  a  few  c.c.  of  the  liquid,  and  allowed  to  settle  in  a  bottle  filled  to 
the  neck.  The  clear  liquid  is  then  used. 

The  gases  present  will 'be  nitrogen,  oxygen,  and  traces  of  am- 
monia. If  hydrocyanic  acid  is  found  in  the  liquid  before  boiling, 
ammonic  cyanide  vapor  may  be  present  in  the  collected  gases. 

Before  collecting  the  gases,  about  1  c.c.  of  caustic  potash  solution 
is  admitted  to  the  nitrometer,  and  mixed  with  the  condensed  water 
in  the  collecting  tube  of  the  same.  The  gas  is  then  transferred  to 
the  second  nitrometer,  which  contains  a  drop  of  sulphuric  acid. 
This  absorbs  the  ammonia.  The  gas,  now  cooled  and  measured,  con- 
sists of  nitrogen  and  oxygen ;  it  is  transferred  to  a  third  nitrometer, 
containing  alkaline  pyrogallate.  After  the  oxygen  is  absorbed,  the 
residual  nitrogen  is  transferred  to  the  measuring  nitrometer,  cooled, 
and  the  volume,  etc.,  noted.  The  loss  in  volume  is  then  calculated 
to  normal  temperature  and  pressure,  from  which  is  deduced  the 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  97 

number  of  c.c.  oxygen  at  normal  temperature  (0°  C.)  and  pressure 
(760  mm.)  per  liter  of  cyanide  solution. 

Bettel  states  that  the  following  decompositions  occur  when  cyan- 
ides and  cyanates  are  boiled  under  the  conditions  of  the  test : 

(1)  MCN  +  2H20  =  MHC02  -f  NH3. 

(2)  2MCNO  +  4H20  =  M2C03  +  (NH4)2C03. 

He  also  says  that  the  dissolved  oxygen  does  not  perceptibly  attack 
the  formates,  sulphocyanides,  ferrocyanides,  nickel,  cobalt,  copper 
and  zinc  compounds  in  solution. 

METHOD  No.  2. 

Estimation  of  Oxygen  by  a  Modification  of  Thresh's  lodometric 
Method. 

This  method  is  fully  described  by  A.  F.  Crosse  (Journal  Chem. 
and  Met.  Soc.  South  Africa,  1899,  pp.  107-112).  See  also,  for  de- 
tails of  Thresh's  process,  Sutton  (Volum.  Anal.,  8th  ed.,  pp.  305- 
310)  and  Journal  Chem.  Soc.  (Vol.  LVIL,  p.  185). 

It  is  based  on  the  fact  that  when  potassium  iodide  and  a  nitrate 
are  added  to  water,  which  has  been  acidulated  with  sulphuric  acid, 
iodine  is  liberated  in  addition  to  the  amount  due  to  the  reaction : 

(a)  H2S04  +  KI  +  KN02  =  K2S04  +  H20  +  NO  +  I, 
this  additional  decomposition  arising  as  follows: 

(&)  3H2S04  +  4KI  +  2KN02  +  0  =  3K2S04  +  3H20  +  2NO 
+  2I2. 

From  these  reactions  it  will  be  seen  that  16  parts  of  oxygen  liber- 
ate 2  X  127  parts  of  iodine  (deducting  the  iodine  due  to  reac- 
tion a). 

The  solutions  required  are : 

1.  Combined  nitrite  and  iodide  solution  consisting  of: 
Sodium  nitrite,  0.5  gram ; 

Potassium  iodide,  20  grams; 
Distilled  water,  100  c.c. 

2.  Dilute  sulphuric  acid: 

Pure  concentrated  H2S04,  1  part; 
Distilled  water,  3  parts. 

3.  Clear  fresh  solution  of  starch. 

4.  Sodium  thiosulphate,  7.75  grams  in  1  liter;  1  c.c.  corresponds 

to  0.25  milligram  of  oxygen. 

5.  Zinc  sulphate,  200  grams  ZnS04.7H30  per  liter. 


98  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

6.  Bromine  water,  consisting  of 
Bromine,  1  part; 
Water,  2  parts. 

The  operation  is  conducted  in  an  atmosphere  free  from  oxygen, 
e.g.,  of  coal  gas.  This  may  be  conveniently  arranged  by  using  a 
wide-mouthed  jar  of  about  500  c.c.  capacity,  closed  by  a  rubber 
stopper  having  four  perforations :  two  of  these  are  for  the  entrance 
and  exit  of  the  gas,  one  to  receive  a  separating  funnel  through 
which  the  solution  to  be  tested  is  introduced,  after  addition  of  the 
necessary  reagents,  and  the  fourth  to  admit  the  thiosulphate  solu- 
tion for  titrating  the  liberated  iodine. 

The  separating  funnel  has  a  stopper  at  the  upper  and  a  tap  at  the 
lower  end,  and  its  capacity  must  be  accurately  determined. 

Preliminary  Tests. — In  the  method  as  modified  by  Crosse  the  solu- 
tion is  first  prepared  by  removal  of  all  cyanides  and  absorbents  of 
iodine.  For  this  purpose  the  solution  to  be  examined  is  first  treated 
with  zinc  sulphate.  A  bottle  capable  of  holding  2£  liters  of  the 
liquid  to  be  tested  is  carefully  filled  and  well  stoppered,  its  exact 
capacity  being  known. 

Test  for  Zinc  Required  to  Precipitate  Cyanide. — One  hundred  c.c. 
are  withdrawn  from  the  large  bottle  and  titrated  with  the  zinc  sul- 
phate solution  (No.  5  above),  using  phenol  phthalein  as  indicator, 
until  the  tint  is  just  destroyed.  The  calculated  quantity  of  zinc 
sulphate  is  then  added  to  the  large  bottle  (without  admitting  air), 
the  contents  well  shaken  and  allowed  to  settle. 

Test  for  Iodine  Absorbents. — A  quantity  of  the  prepared  solution 
(siphoned  off  without  allowing  access  of  air)  is  mixed  with  a 
little  of  the  dilute  sulphuric  acid  (No.  2)  and  a  few  drops  of  potas- 
sium iodide  and  starch.  Dilute  bromine  water  (No.  6)  is  added 
until  a  blue  color  is  obtained. 

Test  for  Oxygen. — The  separating  funnel  is  now  filled  with  the 
prepared  solution.  The  same  amount  of  sulphuric  acid  (say,  0.9  c.c. 
for  300  c.c.  of  solution),  as  in  the  previous  test,  and  the  amount  of 
bromine  water  shown  to  be  necessary,  are  now  introduced  and  mixed 
by  closing  the  stopper  and  turning  the  vessel  over  several  times. 
Next,  1  c.c.  of  the  nitrite  and  iodide  solution  (No.  1)  are  intro- 
duced, and  the  stopper  immediately  replaced.  The  lower  end  of  the 
funnel  is  now  inserted  through  the  stopper  of  the  wide-mouthed  jar 
described  above,  which  has  previously  been  connected  with  the  gas 
supply  and  with  the  burette  containing  the  standard  thiosulphate. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  99 

The  apparatus  is  filled  with  gas  and  allowed  to  remain  at  rest  for 
15  minutes.  The  stopper  of  the  funnel  is  then  opened,  and  the  tap 
turned  so  that  the  mixture  is  allowed  to  run  into  the  jar.  The 
thiosulphate  is  now  run  in  slowly  through  a  tube  drawn  out  to  a 
rather  fine  point,  and  connected  at  its  upper  end,  by  means  of  rubber 
tubing,  with  the  burette  from  which  the  standard  thiosulphate 
(No.  4)  is  delivered.  When  the  color  of  the  iodine  is  nearly  dis- 
charged, starch  solution  is  introduced  through  the  funnel,  and  the 
titration  continued.  At  first  the  color,  after  being  completely  dis- 
charged, returns  on  standing  for  a  few  seconds. 

Correction  for  Nitrates  in  Solution  and  Reagents  Used. — In  cases 
where  nitrites  are  present  the  process  is  altered  as  follows:  Add 
potassium  hydroxide  and  then  zinc  sulphate;  determine  the  thio- 
sulphate required  by  Thresh's  method  with  clear  solution  after 
settlement  and  decantation;  make  a  qualitative  test  for  nitrites  by 
acidifying  a  little  of  the  clear  solution  with  dilute  sulphuric  acid 
and  adding  potassium  iodide  and  starch,  and  finally  apply  a  correc- 
tion for  the  nitrites  and  reagents  used.  The  test  required  for  this 
correction  is  as  follows:  "Pour  into  a  very  strong  350  c.c.  flask  a 
quantity  of  solution  equal  to  that  used  in  the  experiment  (say, 
293  c.c.),  add  a  few  drops  of  KOH  and  close  the  flask  with  a  rubber 
stopper  having  one  perforation,  through  which  is  passed  a  glass  tube 
with  a  glass  stop-cock.  Boil  the  solution  for  a  few  minutes  and 
close  the  stop-cock.  Cool  the  flask,  and  when  cold  pour  the  liquid 
into  the  funnel  of  Thresh's  apparatus,  add  1  c.c.  of  the  iodide 
and  nitrite  solution  and  1  c.c.  of  sulphuric  acid  (1:1).  Allow  to 
stand  for  10  minutes  and  titrate  in  an  atmosphere  of  coal  gas,  as 
previously  described."  The  quantity  of  thiosulphate  required  gives 
the  correction  for  nitrites  and  for  the  reagents,  as  the  same  amounts 
of  reagents  are  used  in  both  testc. 

Simplified  Process. — The  following  simplified  method  is  described 
by  A.  F.  Crosse  (Journal  Chern.  and  Met.  Soc.  South  Africa,  Vol. 
III.,  p.  4).  A  solution  is  prepared  containing  potassium  iodide,  20 
grams,  potassium  nitrite,  2  grams,  made  up  to  100  c.c. 

The  cyanide  solution  to  be  tested  is  siphoned  into  the  separating 
funnel  without  previous  treatment  with  zinc  sulphate,  then  1  c.c. 
of  the  above  iodide  and  nitrite  solution  is  added,  and  3  c.c.  of  sul- 
phuric acid  (1:1).  After  shaking  up  and  allowing  to  stand  for 
15  minutes,  titrate  as  before  described,  in  an  atmosphere  of  coal  gas, 
with  the  same  thiosulphate  solution  (Xo.  4  above).  There  is  no 


100  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

visible  liberation  of  iodine,  as  the  colorless  iodide  of  cyanogen  is  pro- 
duced in  proportion  to  the  free  oxygen  plus  the  quantity  due  to  the 
reagents.  This  substance  may,  however,  be  titrated  with  thiosul- 
phate  and  starch. 

The  correction  for  nitrites  in  reagents,  etc.,  is  made  as  follows : 
Take  about  400  c.c.  of  the  solution,  and  add  0.3  gram  pure  ferrous 
sulphate  and  the  same  weight  of  caustic  lime;  shake  up  well  and 
filter  into  a  flask  through  which  coal  gas  is  passing.  The  precipi- 
tated ferrous  hydrate  absorbs  all  free  oxygen,  so  that  any  iodine 
liberated  on  testing  the  filtrate,  as  before,  will  be  due  to  the  reagents 
alone. 


SECTION  B. 

ESTIMATION  OF  ACTIVE  HALOIDS. 

The  metallic  compounds  of  the  haloid  elements,  such  as  the 
chlorides,  bromides  and  iodides  of  the  alkali  metals,  exert  little  or 
no  beneficial  effect  on  the  extraction  of  the  precious  metals  by 
cyanide;  but  it  is  well  known  that  the  haloid  elements  themselves, 
when  added  in  suitable  proportions  to  a  cyanide  solution,  form  com- 
pounds which  accelerate  the  rate  of  solution  to  a  remarkable  degree. 
Since  the  use  of  bromide  of  cyanogen  in  conjunction  with  potassium 
cyanide  has  been  adopted  to  some  extent,  as  in  the  Sulman-Teed 
and  Diehl  processes,  we  shall  describe  a  method  for  estimating  this 
body  in  presence  of  cyanides,  etc. 

Estimation  of  Cyanogen  Bromide. — On  adding  a  few  drops  of 
concentrated  hydrochloric  acid,  this  compound  is  decomposed  as 
follows : 

BrCy  +  HC1  =  HCy  +  ClBr. 

If  now  we  add  an  excess  of  potassium  iodide,  a  liberation  of 
iodine  occurs  as  follows : 

ClBr  -f  2KI  =  KC1  +  KBr  +  I2 ; 
or,  expressing  both  changes  in  one  equation : 

BrCy  +  HC1  +  SKI  =  HCy  +  KC1  +  KBr  +  I2. 

Hence  53  parts  of  bromide  of  cyanogen  liberate  127  parts  of 
iodine. 

The  free  iodine  may  be  estimated  in  the  ordinary  way  by  standard 
thiosulphate  and  starch: 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  101 

1  c.c.  N/10  thiosulphate  =  0.0053  gram  BrCy. 
2Na2S203  +  I2  =  2NaI  +  Na2S406. 

The  reaction  in  the  case  of  chloride  of  cyanogen  would  presumably 
be: 

CICy  +  HC1  +  SKI  =  HCy  -f  2KC1  +  I2. 
The  presence  of  free  cyanides  does  not  interfere,  as  the  hydro- 
cyanic acid  liberated  does  not  absorb  iodine  in  presence  of  excess  of 
hydrochloric  acid. 

It  must  be  remembered  that  many  other  oxidizing  agents,  e.g., 
ferricyanides,  peroxides,  persulphates,  and  also  nitrites,  decompose 
potassium  iodide  under  similar  conditions,  so  that  when  these  are 
present  the  test  will  not  give  correct  results. 


SECTION  C. 
ESTIMATION  OF  PEROXIDES. 

METHOD  No.  1. 
By  Liberation  of  Iodine. 

In  the  absence  of  other  oxidizing  agents  the  peroxides  of  sodium 
and  hydrogen  may  be  estimated  by  acidifying  pretty  strongly  with 
sulphuric  acid,  adding  excess  of  potassium  iodide,  allowing  to  stand 
for  about  5  minutes,  and  titrating  the  liberated  iodine  with  N/10 
or  N/100  thiosulphate  and  starch. 

1  c.c.  N/10  thiosulphate  =  0.0017  gram  H202. 
=  0.0039  gram  Na202. 
The  reactions  are: 

H2S04  -f  SKI  =  K2S04  +  2HI. 
2HI  +  H202  =  2H,0  -f  I2. 

Where  the  peroxide  of  an  alkali  metal  is  present,  it,  of  course, 
yields  hydrogen  peroxide  on  acidifying. 

METHOD  No.  2. 
By  Titration  with  Permanganate. 

Where  interfering  substances  are  absent,  we  may  estimate  per- 
oxides by  means  of  the  following  reaction;  the  solution  must  be 
diluted  considerably  if  much  peroxide  is  present: 


102  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

2KMn04  +  5H202  +  3H2S04  = 
K2S04  +  2MnS04  +  8H20  +  502. 

A  more  exact  method  is  to  add  an  excess  of  permanganate,  and 
determine  the  excess  by  standard  oxalic  acid. 


SECTION  D. 
ESTIMATION  OF  FERRICYANIDES. 

Ferricyanides  rarely  occur  in  ordinary  working  solutions.  They 
have  sometimes  been  used,  however,  as  aids  to  extraction,  and  may 
occasionally  be  formed  by  the  reactions  taking  place  during  treat- 
ment. 

1.  When  other  oxidizing  agents  are  absent,  they  may  be  deter- 
mined by  liberation  of  iodine  from  potassic  iodide. 

2.  When  reducing  agents  are  absent,  they  may  be  determined,  after 
reduction  to  ferrocyanide,  by  titration  with  permanganate.     This 
reduction  may  be  performed:  (a)  by  ferrous  sulphate  in  presence 
of  excess  of  alkali;  (&)  by  sodium  peroxide;  (c)  by  sodium  amal- 
gam. 

3.  By  titration  with  copper  sulphate  or  nitrate,  using  ferrous  sul- 
phate indicator. 

METHOD  No.  1. 
Estimation  of  Ferricyanides  by  Liberation  of  Iodine. 

This  method  was  originally  described  by  E.  Lenssen  (Ann.  der 
Chem.  u.  Pharm.,  105,  62),  and  improved  by  C.  Mohr  (see  also 
Fresenius,  Quant.  Anal.,  7th  ed.,  Vol.  L,  p.  379,  and  Sutton,  Volum. 
Anal.,  8th  ed.,  p.  227). 

It  is  carried  out  as  follows :  A  measured  quantity  of  the  solution 
is  taken,  potassium  iodide  crystals  added,  together  with  hydrochloric 
acid  in  tolerable  quantity ;  then  a  solution  of  pure  zinc  sulphate  in 
excess.  After  standing  for  a  few  minutes,  till  the  decomposition  is 
complete,  the  excess  of  acid  is  neutralized  by  bicarbonate  of  soda  in 
slight  excess.  The  liberated  iodine  may  then  be  titrated  with 
N/10  thiosulphate  and  starch,  with  great  exactness.  The  reaction  is 
as  follows: 

K3FeCy.  +  KI  =  K4FeCy6  +  I. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  103 

The  ferrocyanide  thus  formed  is  precipitated,  on  addition  of  zinc 
sulphate,  as  ferrocyanide  of  zinc. 

Free  cyanides  appear  to  interfere  slightly  with  the  reaction, 
rendering  the  results  somewhat  too  low. 


METHOD  No.  2. 

Estimation  of  Ferricyanides  by  Reduction  to  Ferrocyanides  and 
Titration  with  Permanganate. 

Ferrocyanides,  thiocyanates,  etc.,  must  be  absent,  or,  if  present, 
their  amount  must  be  determined  by  a  separate  experiment  and 
allowed  for. 

(a)  Reduction  with  Ferrous  Salt. — According  to  Sutton 
(Volum.  Anal.,  8th  ed.,  p.  227),  the  reduction  is  best  performed  by^ 
boiling  with  an  excess  of  caustic  alkali  and  adding  small  quantities 
of  concentrated  solution  of  ferrous  sulphate  until  the  precipitate 
which  occurs  possesses  a  blackish  color  (signifying  that  the  magnetic 
oxide  is  formed)  : 
3Fe(OH)2  +  2K3FeCye  +  2KOH  =  2K4FeCye  +  Fe3(OH)8. 

The  solution  is  diluted  if  necessary,  filtered  through  a  dry  filter, 
and  an  aliquot  part  acidulated  with  sulphuric  acid  and  titrated  as 
described  under  ferrocyanides. 

(&)  Reduction  with  Peroxides. — Another  method  consists  in  boil- 
ing with  excess  of  potash,  then  cooling  and  adding  hydrogen  per- 
oxide till  the  color  is  yellow, 

2K3FeCy6  -f  2KOH  +  H202  =  2K4FeCy6  +  2H20  +  02, 
or  heating  with  sodium  peroxide  till  the  effervescence  ceases,  boiling 
off  excess  of  peroxide,  acidifying  with  sulphuric  acid,  cooling,  and 
titrating  with  permanganate.     (Kassner,  Arch.  Pharm.,  232,  226.) 

(c)  Reduction  with  Sodium  Amalgam. — Bettel  (Proceedings 
Chem.  and  Met.  Soc.  South  Africa,  Vol.  I.,  p.  168)  recommends  al- 
lowing sodium  amalgam  to  act  for  15  minutes  on  the  solution  in  a 
narrow  cylinder,  then  estimating  the  ferrocyanide  formed,  of  course, 
deducting  ferrocyanide  and  thiocyanate  originally  present. 


104  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

CLASS  V. 
INACTIVE  BODIES. 

General  Remarks. — In  addition  to  the  active  cyanogen  compounds 
and  various  substances  which  either  promote  or  interfere  with  their 
action  in  the  treatment  of  ores,  there  are  a  number  of  substances 
which  appear  to  have  little  or  no  influence  on  the  result.  They, 
may  arise  as  impurities  in  the  commercial  cyanide  used,  or  may  be 
derived  from  the  material  treated. 

Among  the  many  substances  of  this  class  which  may  occur,  the 
following  will  be  here  considered: 

1.  Cyanates  and  isocyanates.         3.  Nitrates. 

2.  Chlorides.  4.  Sulphates. 

5.  Silicates. 


SECTION  A. 
ESTIMATION  OF  CYANATES  AND  ISOCYANATES. 

Salts  of  cyanic  acid  appear  to  be  almost  invariably  present  in 
samples  of  commercial  cyanide.  They  exist  in  two  isomeric  forms, 
probably  corresponding  to  different  molecular  formulae,  as  follows : 

Cyanates,     R  —  0  —  C  ^N. 

Isocyanates,  R  —  N  =  C  =  0. 

They  exhibit  the  following  difference  in  properties;  whereas  the 
alkaline  cyanates  are  not  precipitated  by  silver  nitrate,  the  iso- 
cyanates are  completely  precipitated,  thus : 

KNCO  +  AgN03  =  AgNCO  +  KN03. 

METHOD  No.  1. 
By  Precipitation  with  Silver  Nitrate  and  Chromate  Indicator. 

The  reaction  given  above  is  the  basis  of  the  following  method, 
described  by  Feldtmann  and  Bettel  (Proceedings  Chem.  and  Met. 


CHEMISTRY    OF    CYANIDE    SOLUTION'S.  105 

Soc.  of  South  Africa,  Vol.  I.,  p.  272).  It  only  serves  for  the  esti- 
mation of  isocyanates. 

If  to  a  solution  containing  cyanide,  isocyanate,  sulphocyanide, 
ferrocyanide,  chloride,  carbonate  and  bicarbonate  we  add  nitrate  of 
silver  in  excess,  we  get  a  precipitate  consisting  of  silver  cyanide, 
silver  isocyanate  (carbimide),  silver  sulphocyanide  (thiocyanate), 
silver  ferrocyanide,  silver  chloride,  silver  carbonate. 

By  adding  cold  carbonic  acid  water  to  the  solution,  the  alkaline 
carbonate  is  converted  into  bicarbonate,  which  does  not  produce  a 
precipitate  with  silver  nitrate. 

Having  estimated  chlorides,  cyanides,  sulphocyanides  and  ferro- 
cyanides,  and  determined  their  precipitation-value  in  terms  of  c.c. 
of  silver  nitrate  solution  (one  need  hardly  remark  that  sulphides, 
if  present,  are  first  removed  from  solution  by  lead  carbonate),  the 
actual  titration  for  isocyanates  is  now  performed  after  addition  of 
excess  of  carbonic  acid  water  and  two  drops  of  chromate  of  potash 
(see  Vielhaber's  method  for  cyanide,  above). 

The  chromate  solution  should  be  added  only  after  the  reaction 
with  silver  nitrate  is  nearly  complete.  A  little  care  is  necessary  in 
judging  the  change  of  color,  a  comparison  solution  tinted  with 
chromate  being  used  as  a  guide  to  the  end  reaction.  According  to 
Bettel  and  Feldtmann,  the  titration  must  further  be  made  in  ice- 
cold  water,  as  the  isocyanate  appears  to  decompose  pretty  rapidly  at 
ordinary  temperatures,  being  probably  converted  into  the  normal 
cyanate,  which,  as  before  stated,  is  not  precipitated  by  nitrate  of 
silver. 

The  presence  of  ferrocyanides,  thiocyanates,  chlorides,  etc.,  does 
not  interfere,  but  their  equivalent  in  c.c.  of  silver  solution  must  be 
deducted  from  the  number  obtained  in  titrating  for  isocyanate. 

Using  a  solution  containing  13.039  grams  AgN03  per  liter,  the 
silver-equivalents  (for  complete  precipitation)  will  be  as  follows : 


1  c.c.  standard  AgN03  = 


0.005  gram  KCy. 
0.0071  gram  K4FeCy6. 
0.0075  gram  KCNS. 
0.00623  gram  KNCO. 
0.002723  gram  Cl. 


106  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

METHOD  No.  2. 

Estimation  of  Cyanates  by  Differential  Method,  Using  Silver  Ni- 
trate with  and  without  Addition  of  Nitric  Acid. 

The  following  method,  due  to  E.  Victor  (Zeitschrift  f.  Anal. 
CJiem.,  1901,  40  [7]  462-465;  see  also  Journal  Soc.  Chem.  Ind., 
Vol.  XX.,  p.  1031),  is  based  on  the  fact  that  silver  nitrate  pre- 
cipitates cyanide  and  cyanate  completely  in  neutral  solution,  but 
only  cyanide  in  presence  of  nitric  acid,  silver  cyanate  being  readily 
soluble  in  that  acid. 

An  approximately  10  per  cent  solution  of  the  sample  of  commer- 
cial cyanide  is  prepared,  and  of  this,  two  portions  of  10  c.c.  each  are 
measured  into  100  c.c.  flasks,  to  each  of  which  a  known  excess  of 
N/10  AgN03  is  added.  One  lot  is  then  diluted  to  the  mark,  fil- 
tered, and  the  excess  of  silver  titrated  in  an  aliquot  portion  of  the 
liquid  after  acidification  with  HN03,  by  N/10  ammonium  thio- 
cyanate,  using  ferric  alum  as  indicator  (i.e.,  by  Volhard's  method). 
The  total  cyanide  and  cyanate  is  thus  found.  To  the  other  portion 
10  c.c.  of  dilute  nitric  acid  is  added,  the  liquid  diluted  to  100  c.c., 
filtered,  and  the  excess  of  silver  titrated  as  before. 

The  silver  consumed  corresponls  to  the  cyanide  in  the  solution. 
[If  ferrocyanides,  thiocyanates  or  chlorides  were  originally  present 
they  would  not  interfere,  as  they  would  affect  both  titrations 
equally.] 

Carbonates,  if  present,  must  be  decomposed  by  addition  of 
barium  nitrate  before  proceeding  as  above.  [According  to  W.  J. 
Mellor  (see  below)  barium  nitrate  cannot  be  used  on  account  of  the 
insolubility  of  barium  cyanate,  and  calcium  nitrate,  free  from 
chlorides,  is  recommended.] 

The  author  refers  to  the  process  of  Feldtmann  and  Bettel,  given 
above,  according  to  whom:  (a)  Only  isocyanate  and  not  normal 
cyanate,  gives  an  insoluble  silver  salt;  (&)  an  isocyanate,  in  solution 
is  rapidly  transformed  into  normal  cyanate  at  quite  low  tempera- 
tures (e.g.,  25°  C.). 

For  these  reasons  they  (Feldtmann  and  Bettel)  insist  that  the 
solution  of  the  sample  and  the  precipitation  with  silver  nitrate  must 
be  effected  at  a  temperature  near  0°  C.  in  order  to  avoid  loss  of  iso- 
cyanate. 

E.  Victor  has  examined  these  statements  experimentally,  and 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  107 

finds  that  a  solution  of  alkaline  isocyanate  does  not  change  sensibly 
at  25°  C.  in  three  hours.  Only  after  24  hours'  standing  a  diminution 
of  about  30  per  cent  had  taken  place,  the  loss  being  due  not  to  con- 
version of  isocyanate  into  normal  cyanate,  but  to  the  well-known  de- 
composition of  the  iso-salt  into  ammonia  and  potassium  carbonate: 
KNCO  +  2H20  =  KHC03  +  NH3. 

In  any  case  there  is  no  necessity  to  operate  at  such  inconveniently 
low  temperatures. 

The  reaction  of  nitric  acid  on  silver  cyanate  appears  to  be 
AgCNO  -h  2HN03  -f-  H20  =  AgN03  +  NH4N03  -f  C0a. 


METHOD  No.  3. 

Estimation  of  Cyanate  by  Means  of  Acid  Required  to  Dissolve  Silver 
Cyanate. 

A  method  very  similar  to  the  preceding  is  given  by  J.  W. 
Mellor  (Zeitschrift  f.  Anal  Chem.,  1901,  34  [1]  17-21  and  Journal 
Soc.  Chem.  Ind.,  Vol.  XX.,  p.  284;  see  also  Allen  Comm.  Org. 
Anal.,  Vol.  III.,  p.  484). 

1.  Twenty  grams  of  the  commercial  cyanide  are  dissolved  in  about 
100  c.c.  of  water  (or  a  convenient  volume  of  solution  is  measured 
out),  and  the  carbonates  precipitated  by  calcium  nitrate  free  from 
chlorides.    Barium  nitrate  cannot  be  used,  as  barium  cyanate  is  in- 
soluble.    The  precipitate  is  filtered  off,  washed,  and  the  filtrate  and 
washings  made  up  to  200  c.c.  (A)     One  c.c.  of  solution  =  0.1  gram 
of  commercial  cyanide. 

2.  Ten  c.c.  of  the  solution  (A)  are  treated  with  excess  of  am- 
monia solution  and  a  few  drops  of  KI,  and  titrated  for  cyanide  in  the 
ordinary  manner  with  silver  nitrate. 

3.  To  about  10  c.c.  of  the  solution  (A),  silver  nitrate  is  added 
until  no  further  precipitation  takes  place.    The  precipitate,  con- 
sisting of  a  mixture  of  silver  cyanide  and  silver  cyanate,  is  collected 
on  a  filter  and  washed  with  ice-cold  water. 

It  is  then  heated  to  about  50°  C.  with  about  5  c.c.  of  normal  nitric 
acid,  which  decomposes  the  whole  of  the  cyanate.  The  filtrate  is 
titrated  with  normal  sodium  hydroxide.  Assuming  n  c.c  to  be 
required,  1  gram  of  original  sample  would  contain  0.0405  (5  —  n) 
gram  KCNO. 

According  to  0.  Herting  (Journal  Soc.  Chem.  Ind.,  Vol.  XX.) 


108  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

the  temperature  of  50°  C.  is  insufficient;  he  states  that  more 
reliable  results  are  obtained  by  digesting  on  a  water  bath  with  the 
nitric  acid  for  at  least  an  hour. 


METHOD  No.  4. 
Estimation  of  Cyanates  by  Determination  of  Nitrogen. 

0.  Herting  (Zeitschrift  f.  Angew.  Chem.,  1901  [24],  585-586) 
recommends  the  following  method,  based  on  the  facts  that  cyanides 
are  decomposed  by  acids  with  evolution  of  HCy  and  formation  of 
salts  of  the  metal,  whereas  cyanates  are  decomposed  according  to  the 
equation : 

RCNO  +  2HC1  +  H20  =  EC1  +NH4C1  +  C02. 

In  making  a  determination,  from  0.2  to  0.5  gram  of  the  com- 
mercial cyanide  is  dissolved  in  a  few  c.c.  of  water,  and  the  solution 
evaporated  with  HC1  or  H2S04  on  the  water-bath.  The  residue  is 
dissolved  in  water,  and  the  nitrogen  in  the  solution  is  determined 
as  NH3  by  distillation  with  sodium  hydroxide  and  calculated  as 
KCNO.  (See  estimation  of  ammonia,  above.) 

Qualitative  Test  for  Cyanates. — Small  quantities  of  cyanate  may 
be  detected  in  potassium  cyanide  by  the  use  of  cobalt  acetate.  This 
gives  an  intense  blue  color,  forming  a  compound  which  crystallizes 
as  Co(CNO)2-}-2KCNO. 

The  cyanide  is  dissolved  in  a  minimum  quantity  of  water,  abso- 
lute alcohol  added  to  precipitate  most  of  the  KCy,  then  carbonic 
acid  is  passed  into  the  remaining  liquid  for,  say,  45  minutes,  the 
K2C03  formed  is  filtered  off,  and  the  filtrate  tested  with  cobalt  acetate 
(Engineering  and  Mining  Journal,  Vol.  LX.,  p.  489). 


SECTION  B. 
ESTIMATION  OP  CHLORIDES. 

The  estimation  of  chlorides  in  cyanide  solutions  may  be  made  in 
several  ways,  most  of  which  involve  precipitation  as  silver  chloride. 
The  following  will  be  here  considered : 

1.  Precipitation  with  silver  nitrate  and  chromate  indicator,  mak- 
ing corrections  for  cyanogen  compounds. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  109 

2.  Precipitating  cyanide  and  chloride  together  as  silver  salts, 
and  determining  the  former  in  the  precipitate  by  organic  analysis. 

3.  Precipitating  cyanide  and  chloride  together  with  silver,  de- 
composing with  sulphuric  acid  and  zinc,  and  estimating  chlorine 
in  the  liquid. 

4.  Decomposing  silver  precipitate  by  fusion  with  carbonate  of 
soda  and  niter. 

5.  Evaporating  with  nitric  acid,  and  fusing  residue  with  sodium 
carbonate  and  niter. 


METHOD  No.  1. 
Estimation  of  Chlorides  by  Direct  Precipitation  with  Silver  Nitrate. 

This  is  merely  an  application  of  Mohr's  well-known  volumetric 
method  of  estimating  chlorides,  and  has  been  already  described 
under  total  cyanogen.  The  other  constituents  of  the  solution, 
which  are  precipitated  by  silver  nitrate  before  the  chromate  reaction 
occurs,  must  be  separately  determined,  and  their  equivalents  in 
c.c.  of  standard  silver  nitrate  solution  deducted  from  the  result  of 
the  titration.  Carbonates  may  be  removed  by  preliminary  treatment 
with  carbonic  acid  water,  as  in  Bettel  and  Feldtmann's  method  for 
cyanates  (see  above),  or  the  protective  alkali  may  be  neutralized  by 
cautious  addition  of  dilute  nitric  acid. 

It  must  be  remembered  that  free  cyanides  are  completely  precipi- 
tated as  AgCy,  using  twice  as  much  AgN03  as  would  be  required  to 
give  a  permanent  turbidity,  and  that  ferrocyanides  and  thiocyanates 
are  completely  precipitated  as  silver  salts  before  the  chromate  re- 
action occurs. 

Using  a  solution  containing  13.039  grams  AgN03  per  liter,  the 
silver  equivalents  for  complete  precipitation  will  be  as  follows : 


1  c.c.  standard  AgN03  =  • 


0.005  gram  KCy. 
0.002723  gram  Cl. 
0.00449  gram  NaCl. 
0.00572  gram 


HO  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

METHOD  No.  2. 

Estimation  of  Chlorides  by  Precipitating  with  Cyanides,  as  Silver 
Salts,  and  Decomposing  Precipitate  to  Determine  Cyanides. 

This  method  is  given  by  Fresenius  (Quant.  Anal.,  7th  ed.,  Vol.  I., 
p.  512)  as  follows: 

Precipitate  with  solution  of  silver,  collect  the  precipitate  upon 
a  weighed  filter,  and  dry  on  the  water  bath  until  the  weight  re- 
mains constant;  then  determine  the  cyanogen  by  the  method  of  or- 
ganic analysis  (e.g.,  by  heating  with  copper  oxide  or  lead  chromate 
in  a  combustion  tube  and  collecting  the  nitrogen  evolved  over  mer- 
cury and  caustic  potash,  or  by  conversion  into  ammonia  by  distilla- 
tion with  soda  lime.  The  chlorine  is  found  by  difference. 

This  method  would  require  further  modifications  in  presence  of 
ferrocyanides,  thiocyanates,  etc.,  and  in  any  case  would,  in  general, 
be  too  tedious  for  use  in  the  analysis  of  cyanide  solutions. 

METHOD  No.  3. 

Precipitation  of  Chloride  and  Cyanide  Together  as  Silver  Salts, 
Decomposing  the  Precipitate  and  Estimating  Chlorine. 

This  method  is  given  in  Fresenius  (Quant.  Anal.,  7th  ed.,  p. 
512)  and  Allen  (Comm.  Org.  Anal,  Vol.  XIL,  pt.  3,  p.  431). 

The  cyanide  and  chloride  are  precipitated  together  as  silver  salts 
by  adding  excess  of  silver  nitrate.  The  precipitate  (zAgCy  + 
vAgCl)  is  dried  at  100°  C.  and  weighed. 

Then  heat  the  precipitate,  or  an  aliquot  part  of  it,  in  a  porcelain 
crucible,  with  cautious  agitation  of  the  contents,  to  complete  fusion ; 
cool,  add  dilute  sulphuric  acid  to  the  fused  mass,  then  reduce  by 
zinc,  filtering  the  solution  from  metallic  silver  and  paracyanide  of 
silver.  The  cyanides,  ferrocyanides,  thiocyanates,  etc.,  will  be  en- 
tirely decomposed,  and  the  whole  of  the  chlorine  will  be  present  in 
the  filtrate  as  chloride  of  zinc.  In  this  liquid  it  may  be  determined 
by  any  of  the  ordinary  gravimetric  or  volumetric  methods.  The 
results  are  said  to  be  very  satisfactory. 

[Allen  observes  that  small  quantities  of  cyanide  in  a  mixture 
with  chloride  are  better  determined  as  in  the  previous  method,  by 
igniting  the  precipitate  with  soda-lime  or  heating  it  with  strong 
sulphuric  acid,  and  determining  the  resultant  ammonia.] 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  Ill 

i 

K.  Kraut  (Zeitschrift  f.  Anal.  Chem.,  2,  p.  243)  decomposes 
the  precipitate  of  silver  cyanide  and  chloride,  after  weighing  as 
above,  by  heating  it,  or  an  aliquot  part  of  it,  with  nitric  acid  of 
gp.  gr.  1.2  in  a  sealed  tube  at  100°  C.  for  several  hours,  or  at  150°  C. 
for  one  hour.  The  cyanide  of  silver  is  completely  decomposed, 
while  the  chloride  is  unaffected.  Filter  the  contents  of  the  tube, 
wash  the  precipitate^  and  weigh  it  as  AgCl.  The  loss  indicates  the 
amount  of  AgCy. 

METHOD  No.  4. 

Decomposition  of  Silver  by  Fusion  with  Sodium  Carbonate  and 
Niter. 

The  solution  is  acidified  with  nitric  acid,  and  precipitated  with 
silver  nitrate  solution;  the  precipitate  filtered  and  washed,  then 
fused  with  4  parts  of  sodium  carbonate  and  1  part  potassium  nitrate. 
The  fused  mass  is  extracted  with  water,  and  the  chlorine  deter- 
mined in  the  solution  by  the  ordinary  gravimetric  or  volumetric 
methods.  (See  Fresenius,  Quant.  Anal.,  7th  ed.,  Vol.  I.,  p.  513.) 

METHOD  No.  5. 

Estimation  of  Chlorides  by  Evaporation  with  Nitric  Acid  and  Fu- 
sion of  Kesidue  with  Sodium  Carbonate  and  Niter. 

Bettel  and  Feldtmann  (Proceedings  Chem.  and  Met.  Soc.  South 
Africa,  Vol.  I.,  p.  275)  gives  the  following  method  for  determining 
chlorides  in  commercial  cyanide:  "A  portion  of  the  cyanide  solu- 
tion, equal  to  about  5  grams  of  the  solid  cyanide,  is  evaporated  with 
sufficient  nitric  acid  to  form  nitrates  with  the  bulk  of  the  potas- 
sium and  sodium  present,  leaving  about  50  to  100  milligrams  of 
cyanide  unattacked.  When  most  of  the  hydrocyanic  acid  has  es- 
caped, about  5  grams  sodium  nitrate  and  3  grams  sodium  carbonate 
are  added,  and  intimately  mixed  with  the  partly  decomposed  cyanide 
solution,  the  evaporation  continued  to  dryness,  and  then  the  residue 
gently  ignited  to  decompose  nitrates  of  iron,  etc.  The  melt  is  now 
cooled,  dissolved  in  water,  and  the  insoluble  oxides  filtered  off  and 
washed.  [The  oxide  of  iron  may  then  be  estimated  and  calculated 
to  ferrocyanide  if  required.]  The  filtrate  from  the  insoluble  oxides 


112  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

is  now  acidulated  with  pure  nitric  acid,  and  the  solution  raised  to 
boiling  point ;  this  decomposes  cyanates  with  formation  of  ammonia 
and  carbon  dioxide.  The  chloride  may  now  be  estimated  in  the 
usual  manner. 

J.  W.  Mellor  (Zeitschrift  f.  Anal  Chem.,  1901,  34  [1],  17-21) 
applies  the  same  method,  fusing  the  commercial  cyanide  direct,  after 
weighing  a  suitable  quantity,  with  a  mixture  of  potassium  nitrate 
and  sodium  carbonate  (1:5),  in  a  porcelain  crucible.  The  mass 
is  dissolved  in  boiling  water,  acidified  with  nitric  acid,  and  the 
chlorides  determined  in  the  usual  way. 


SECTION  C. 
ESTIMATION  OF  NITRATES. 

The  ordinary  methods  for  the  estimation  of  nitrates  are  some- 
what complex  and  involve  delicate  manipulations.  They  are  mostly 
based  upon  conversion  into  ammonia  or  ammonium  salts  by  the 
action  of  strong  acids  and  alkalis,  with  subsequent  distillation  and 
collection  of  the  ammonia.  As  cyanide  solutions  contain  various 
other  substances  capable  of  yielding  ammonia  under  these  condi- 
tions, the  methods  could  not  be  directly  applied.  Probably  most 
of  the  interfering  substances  could  be  removed  by  precipitating 
with  a  pure  solution  of  acetate  or  sulphate  of  silver  in  slight  excess. 
The  filtrate  might  then  be  concentrated  and  distilled  with  the  addi- 
tion of  strong  caustic  potash  and  a  mixture  of  finely-divided  zinc 
and  iron.  (See  Sutton,  Volum.  Anal,  8th  ed.,  p.  274.) 


SECTION  D. 
ESTIMATION  OF  SULPHATES. 

The  estimation  of  sulphates  is  perhaps  best  made  by  the  ordinary 
gravimetric  method  with  barium  chloride,  after  complete  decom- 
position of  the  solution  by  evaporating  several  times  to  dryness  with 
nitric  and  hydrochloric  acids,  finally  with  the  latter  alone.  The 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  113 

residue  is  diluted  with  distilled  water,  filtered,  if  necessary,  a  little 
pure  hydrochloric  acid  added,  heated  nearly  to  boiling,  and  barium 
chloride  added  in  slight  excess.  After  standing  several  hours  the 
precipitate  is  washed  carefully  by  decantation  several  times  with  hot 
water,  finally  collected  on  a  small  filter,  washed  with  hot  water  till 
free  from  chlorides,  dried,  gently  ignited  and  weighed  as  BaS04. 
The  precipitate  must  be  ignited  as  far  as  possible  apart  from  the 
filter  paper.  (For  further  details  see  Fresenius,  Quant.  Anal.,  7th 
ed.,  Vol.  I.,  p.  299.) 

W.  J.  Sharwood  (Engineering  .and  Mining  Journal,  1898,  p. 
216)  recommends  adding  excess  of  hydrochloric  acid  to  the  cyanide 
solution,  heating  till  odor  of  hydrocyanic  acid  has  disappeared  and 
filtering  off  any  zinc  or  copper  ferrocyanides,  Prussian  blue  or  silver 
chloride  that  may  be  precipitated.  Barium  chloride  may  then  be 
added  to  the  filtrate  without  evaporation  to  dryness. 


SECTION  B. 
ESTIMATION  OF  SILICATES. 

Soluble  silicates  of  the  alkali  metals  are  decomposed  by  evapora- 
tion with  nitro-hydrochloric  acid,  and  gentle  ignition  of  the  residue. 
In  some  cases  the  insoluble  residue,  after  this  treatment,  must  be  di- 
gested with  ammonia  and  ammonium  acetate  in  order  to  dissolve 
chloride  of  silver  and  sulphate  of  lead.  The  residue,  which  should 
be  perfectly  white,  is  then  washed  till  no  more  traces  of  iron,  etc., 
are  found  in  the  filtrate,  dried,  ignited  and  weighed  as  Si02. 


114  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

CLASS  VI. 
NOBLE  METALS. 

General  Eemarks. — The  correct  estimation  of  the  gold  and  silver 
contents  of  the  solutions  before  and  after  precipitation  is  very  es- 
sential, as  a  check  on  the  extraction  and  as  evidence  of  efficient  pre- 
cipitation. Some  rapid  method  of  making  these  estimations  is 
much  to  be  desired,  but  at  present,  for  gold,  at  least,  there  is  no  ac- 
curate method  which  does  not  involve  a  fire  assay. 

We  shall  here  describe : 

(A)  Methods  in  which  gold  and  silver  are  determined  together. 

(B)  Methods  in  which  gold  only  is  determined. 
(0)  Methods  in  which  silver  only  is  determined. 


SECTION  A. 
ESTIMATION  OF  GOLD  AND  SILVER  TOGETHEB. 

A  considerable  number  of  methods  have  been  suggested.  These 
may  be  divided  into  the  following  groups : 

(a)  Those  involving  the  evaporation  of  the  entire  quantity  of 
solution  taken  for  the  test. 

(6)  Those  in  which  copper  salts  are  used  as  a  precipitant. 

(c)  Those  in  which  sulphuretted  hydrogen  is  used  as  a  precipi- 
tant. 

(d)  Those  depending  on  reduction  by  other  metals. 

GROUP  (a). — ESTIMATION  OF  GOLD  AND  SILVER  BY  EVAPORATION. 

METHOD  No.  1. 
Evaporation  with  Litharge. 

A  measured  volume  of  the  liquid,  varying  according  to  the  rich- 
ness of  the  solution  from,  say,  30  c.c.  up  to  3  or  4  liters,  is  evap- 
orated at  a  gentle  heat  in  a  porcelain  basin  without  boiling.  The 
solution  may  conveniently  be  measured  on  the  assay-ton  system.  In 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  115 

an  ordinary  case  10  A.T.  or  291.6  c.c.  would  be  a  suitable  quantity. 
Previous  to  evaporation,  from  20  to  50  grams  of  litharge  are  sprinkled 
uniformly  over  the  surface  of  the  liquid.  Some  operators  prefer  to 
mix  a  little  fine  silica,  or  silica  and  charcoal,  with  the  litharge.  It 
is  not  advisable  to  add  carbonate  of  soda,  as  this  forms  a  very  hard 
crust,  difficult  to  remove  from  the  dish.  Towards  the  end  of  the 
evaporation  care  must  be  taken  not  to  overheat  the  residue,  as  this 
might  cause  it  to  adhere  too  firmly. 

When  quite  dry  the  residue  is  scraped  out  with  a  clean  spatula, 
and  mixed  with  a  suitable  flux,  which  may  be  varied  according  to  cir- 
cumstances. The  following  may  be  given  as  examples : 

(a)  (b) 

Litharge  (before  evaporation) 30  grams.      45  grams. 

Borax 20      «          10      " 

Carbonate  of  soda 16      «          40      " 

Silica  —      "          25      " 

Charcoal 0.5      "  1      " 

The  last  portions  remaining  on  the  dish  may  be  removed  by  means 
of  a  small  piece  of  filter  paper  slightly  moistened,  the  paper  being 
afterwards  added  to  the  flux.  If  this  fails  to  perfectly  clean  the 
dish,  a  drop  or  two  of  nitric  acid  is  added. 

The  flux  containing  the  dry  residue,  well  mixed,  is  now  trans- 
ferred to  a  clay  crucible  and  fused  in  an  ordinary  assay  furnace. 
The  fusion  should  be  complete  in  20  to  30  minutes.  The  charge  is 
generally  very  fusible,  giving  a  transparent,  nearly  colorless  slag, 
but  in  other  cases  the  slag  may  be  stained  owing  to  the  presence  of 
iron,  copper  and  other  impurities  in  the  solution.  The  resulting 
lead  button  is  then  cupelled,  keeping  the  temperature  of  the  muffle, 
especially  when  silver  is  to  be  determined,  at  such  a  low  temperature 
that  feathers  of  litharge  are  formed  on  the  cupel.  The  beads  of 
gold  and  silver  are  then  weighed  and  parted  in  the  ordinary  way. 

This  method,  although  somewhat  long,  is  the  most  reliable  and 
accurate  which  has  so  far  been  suggested.  Duplicate  assays  give 
identical  results  when  the  operations  are  properly  carried  out. 

METHOD  No.  2. 
Evaporation  on  Lead  Foil. 

For  comparatively  rich  solutions  it  is  sometimes  convenient  to  de- 
termine the  gold  and  silver  contents  by  evaporating  a  weighed  or 


116  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

measured  quantity  in  a  small  dish  of  lead  foil,  which  may  afterwards 
be  rolled  up  and  cupelled.  Sometimes  the  lead  containing  the 
evaporated  residue  may  be  cupelled  direct,  but  in  other  cases,  owing 
to  the  presence  of  impurities,  it  is  necessary  to  scorify.  When  poor 
solutions  have  to  be  assayed,  large  quantities  of  liquid  must  be  taken, 
hence  either  larger  dishes  of  lead  foil  or  a  number  of  small  dishes 
must  be  used,  the  resulting  lead  buttons  being  afterwards  run  to- 
gether and  reduced  in  weight  by  scorification,  thus  increasing  the 
labor  and  multiplying  the  chances  of  error.  On  the  whole,  except 
for  very  rich  solutions,  the  method  of  evaporation  with  litharge  is 
preferable. 

Alfred  James  (Cyanide  Practice,  1902)  states  that  this  method 
has  a  tendency  to  give  low  results. 

GROUP  (b). — ESTIMATION  OF  GOLD  AND  SILVER  BY  MEANS  OF 
COPPER  SALTS. 

METHOD  No.  1. 
Precipitation  with  Cuprous  Chloride. 

Prof.  S.  B.  Christy  has  shown  that  gold  and  silver  may  be  readily 
precipitated  from  cyanide  solutions  by  means  of  cuprous  chloride. 
It  is  advisable  to  remove  the  bulk  of  the  free  cyanide  by  first  adding 
a  slight  excess  of  sulphuric  or  hydrochloric  acid  and  boiling  for 
some  time.  The  solution  used  for  precipitating  is  prepared  by  add- 
ing hydrochloric  acid  to  a  solution  of  copper  sulphate  and  passing 
in  sulphur  dioxide,  or  by  boiling  the  copper  solution  with  sodium 
sulphite.  On  adding  a  little  of  this  liquid  to  the  hot  acidulated 
cyanide  solution  a  white  precipitate  is  formed,  consisting  chiefly  of 
cuprous  cyanide,  which  carries  down  with  it  the  greater  part  of  the 
gold  and  silver.  After  allowing  to  settle,  the  clear  liquid  is  poured 
off  through  a  filter,  the  precipitate  is  then  collected  on  the  same 
filter,  dried,  scorified  if  necessary  with  grain  lead,  and  cupelled. 
The  resulting  gold  and  silver  bead  is  then  weighed  and  parted. 

It  is  advisable  to  test  the  filtrate,  to  make  sure  that  cuprous  chlo- 
ride is  in  excess.  This  may  be  done  by  filtering  a  little  of  the  liquid 
and  testing  with  potassium  ferrocyanide,  which  gives  the  character- 
istic reddish-brown  precipitate  of  ferrocyanide  of  copper.  The  test 
may  also  be  applied  by  absorbing  a  little  of  the  liquid  with  filter 
paper  and  testing  with  a  drop  of  ferrocyanide  solution  on  a  glass  rod. 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  117 

With  fairly  rich  solutions  the  results  are  satisfactory,  but  the 
precipitation  is  not  absolutely  perfect,  and  gold  and  silver  may  al- 
ways be  found  in  the  filtrate  by  evaporation  with  litharge. 

A.  Whitby  (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol. 
III.,  p.  15)  applies  the  method  as  follows:  To  700  c.c.  of  the 
cyanide  solution  add  25  c.c.  of  a  10  per  cent  solution  of  copper  sul- 
phate, then  5  to  7  c.c.  concentrated  hydrochloric  acid,  and  lastly  10 
to  20  c.c.  of  a  10  per  cent  solution  of  sodium  sulphite.  The  precipi- 
tate, after  vigorous  shaking  for  about  two  minutes,  is  filtered  off  and 
run  down  with  flux  in  the  ordinary  way.  Assays  can  be  completed 
in  three  hours,  and  results  compare  well  with  the  evaporation  test. 
With  slimes  solutions,  which  are  normally  weak  in  cyanide,  it  is 
better  to  add  some  cyanide  before  making  the  test. 

METHOD  No.  2. 
Precipitation  with  Cupric  Sulphate. 

The  following  method  is  given  by  Walter  H.  Virgoe  (Proceedings 
lust.  Min.  and  Met.,  Vol.  X.,  p.  115) : 

To  a  liter  of  the  cyanide  solution  add  excess  of  cupric  sulphate 
and  acidify  with  hydrochloric,  nitric  or  sulphuric  acid ;  the  precip- 
itate, which  is  white  and  flocculent,  contains  all  the  gold  and  silver, 
and  the  bluish  or  greenish  color  of  the  filtrate  indicates  that  excess 
of  the  copper  sulphate  has  been  added.  The  precipitate  is  washed, 
dried  and  placed  in  a  scorifier  with  metallic  lead.  The  results  are 
accurate,  and  the  whole  operation  can  be  done  within  an  hour  and  a 
half. 

The  theory  of  the  method  may  be  expressed  thus : 

2AuKCy2  -f-  Cu2Cy2,  2KCy  +  2H2S04  = 
Au2Cu2Cy4  +  2K,,S04  +  4KCy. 

In  a  subsequent  discussion  on  this  method  by  the  Chemical  and 
Metallurgical  Society  of  South  Africa,  A.  F.  Crosse  recommended  it, 
but  preferred  to  fuse  in  a  crucible  with  litharge,  instead  of  scorifying. 
A.  Whitby  (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol.  III., 
p.  15)  found,  however,  that  all  the  gold  is  not  precipitated  by  copper 
sulphate  with  addition  of  mineral  acid,  but  that  an  additional  precip- 
itate is  obtained  by  mixing  sodium  sulphite  with  the  filtrate,  the 
remainder  of  the  gold  being  quantitatively  recoverable  from  this 
precipitate.  He  therefore  suggests  the  slight  modification  of 
Christy's  method  given  above. 


118  CHEMISTRY    OP    CYANIDE    SOLUTION'S. 

GROUP  (c).— ESTIMATION  OF  GOLD  AND  SILVER  BY  PRECIPITATION 
WITH  SULPHURETTED  HYDROGEN. 

Henry  Watson  (Engineering  and  Mining  Journal,  1898,  p.  753) 
gives  the  following  method,  which  is  stated  to  be  quick,  accurate 
and  economical: 

"A  pint  or  half  a  pint  of  the  cyanide  liquor  is  acidified  with 
HC1,  and  heated  to  boiling.  Add  a  solution  of  2  grams  of  lead! 
acetate,  and  then  pass  in  a  current  of  sulphuretted  hydrogen  gas 
until  all  the  lead  is  precipitated  as  sulphide.  Allow  the  solution  to 
cool  down  somewhat,  so  as  to  thoroughly  saturate  it  with  H2S.  The 
gold  will  be  precipitated  with,  and  collected  by,  the  lead  sulphide. 
Filter  off  the  precipitate  and  dry.  Fuse  and  scorify  the  residue 
and  cupel  the  resulting  lead  button." 

I  may  remark  that  I  tested  a  very  similar  method  many  years  ago, 
but  did  not  find  the  results  at  all  satisfactory,  being  invariably 
lower  than  those  obtained  by  evaporation. 

GROUP  (d). — ESTIMATION  OF  GOLD  AND  SILVER  BY  REDUCTION 
METHOD. 

Precipitation  with  Sodium  Amalgam. 

Sodium  amalgam,  when  shaken  in  a  stoppered  cylinder  with  a 
cyanide  solution  containing  the  precious  metals,  decomposes  the 
double  cyanides  of  these  metals,  the  gold  and  silver  amalgamating 
with  the  mercury.  This  is  then  collected  and  carefully  distilled  at 
a  gentle  heat.  It  is  best  to  cupel  the  residue. 

This  method  does  not  seem  to  be  reliable  with  poor  solutions. 
In  some  cases  not  over  60  per  cent  of  the  gold  value  was  obtained. 


SECTION  B. 
ESTIMATION  OF  GOLD  ALONE. 

Any  of  the  preceding  methods  are  applicable  if  the  silver  be  dis- 
solved by  parting  with  nitric  acid  in  the  usual  way.  The  following 
methods  serve,  however,  in  cases  where  it  is  not  necessary  to  estimate 
the  silver. 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  119 

METHOD  No.  1. 
Precipitation  with  Silver  Nitrate. 

This  method,  due  to  A.  F.  Crosse,  is  as  follows :  The  solution  is 
acidulated  with  nitric  or  sulphuric  acid,  and  boiled  to  expel  most  of 
the  hydrocyanic  acid.  Silver  nitrate  is  then  added  as  long  as  a 
precipitate  forms,  the  solution  allowed  to  settle,  the  clear  liquid 
decanted  through  a  filter,  and  the  precipitate  then  collected  on  the 
same  filter,  dried,  and  the  filter  paper  with  residue  wrapped  in  lead 
foil  and  cupelled.  In  some  cases  it  is  desirable  to  scorify  before 
cupelling,  or  to  fuse  in  a  small  crucible.  For  the  latter  purpose 
the  following  flux  is  recommended : 

Glass,  110  parts.  Soda,  100  parts. 

Litharge,  200  parts.  Argol,  48  parts. 

The  method  gives  fairly  good  results  when  the  solutions  are  not 
too  low  in  gold.  The  precipitate  of  silver  cyanide  carries  down  all 
but  a  small  fraction  of  the  gold,  but  with  solutions  very  weak  in  gold 
the  precipitation  is  not  sufficiently  perfect,  as  may  be  shown  by 
evaporation  of  the  filtrate  with  litharge. 

METHOD  No.  2. 

Estimation  of  Gold  by  Eeduction  with  Zinc  after  Adding  Silver 
Nitrate. 

The  following  description  of  a  method  proposed  by  Buchanan  is 
given  by  M.  Eissler  ('The  Cyanide  Process').  The  object  of  the 
preliminary  addition  of  silver  nitrate  is  by  no  means  obvious. 

"A  known  quantity  of  the  solution  is  precipitated  with  excess  of 
silver  nitrate;  the  precipitate  is  decomposed  by  a  reducing  agent, 
filtered  off,  dried  and  cupelled.  In  practice  the  process  was  carried 
out  as  follows :  195  c.c.  of  the  cyanide  solution  is  transferred  to  a 
flask  of  500  c.c.  capacity,  mixed  with  a  few  drops  of  potassium  chro- 
ma te.  Silver  nitrate  of  any  strength  (say  5  per  cent)  is  then  added 
until  a  reddish  tinge  of  silver  chromate  remains  permanent.  Then 
take  10  to  20  grams  of  zinc  dust  or  shavings,  mix  thoroughly  with 
the  precipitate  and  solution  in  the  flask,  add  2  to  3  c.c.  of  10  per  cent 
sulphuric  acid.  Allow  to  stand  10  minutes ;  add  excess  of  sulphuric 
acid  to  dissolve  the  remainder  of  the  zinc.  Filter,  wash  once,  dry 


120  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

and  incinerate  on  a  roasting  dish  in  muffle,  and  cupel  residue  with  a 
little  lead.  The  results  are  slightly  lower  than  those  obtained  by 
the  ordinary  precipitation  method." 

METHOD  No.  3. 
Estimation  of  Gold  by  Direct  Reduction  with  Zinc. 

The  following  (H.  T.  Durant,  Journal  Chem.  and  Met.  Soc. 
South  Africa,  Vol.  III.,  p.  58)  is  stated  to  be  a  perfectly  accurate 
and  very  rapid  method  of  assaying  gold-bearing  solutions  when 
large  volumes  of  liquid  have  to  be  operated  on. 

"The  solution  for  assay  is  placed  in  a  boiling-flask  of  convenient 
size  and  rendered  strongly  acid  with  sulphuric  acid,  raised  to  the 
boiling  point;  about  30  grams  of  zinc  shavings  are  then  added  in 
quantities  of  about  5  grams  at  a  time,  seeing  that  all  action  ceases 
from  each  addition  of  zinc  before  the  next  addition  of  zinc  is  made. 
The  solution  must  be  kept  strongly  acid  throughout,  adding  more 
acid,  if  necessary,  and  must  also  be  kept  at  boiling  point  through  the 
whole  operation.  After  the  zinc  has  dissolved  there  will  be  a  slight 
residue  of  lead,  carbon,  etc. ;  then  remove  from  source  of  heat  and 
add  about  one  gram  of  precipitated  silica  to  collect,  then  filter 
through  a  double  filter  paper  and  wash  out  flask  well  on  to  the  filter 
paper;  then  fold  up  the  moist  filter  paper  and  place  on  a  layer  of 
borax  in  a  scorifier  to  dry  and  char  slowly  in  front  of  the  muffle; 
when  this  is  complete,  add  granulated  lead  or  litharge  and  reducing 
agent  and  scorify  as  usual,  cleaning  the  slag,  if  necessary.  The 
method  depends  on  the  action  of  nascent  hydrogen,  and  is  by  no 
means  novel,  at  least  in  theory." 


SECTION   C. 
ESTIMATION  OF  SILVER  ALONE. 

METHOD  No.  1. 
Precipitation  as  Sulphide  and  Cupellation. 

A  measured  quantity  of  the  solution,  which  must  be  alkaline,  is 
mixed  with  an  excess  of  sodium  sulphide  solution.  When  only  small 
quantities  of  silver  are  present,  it  is  better  to  add  a  few  drops  of  lead 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  121 

acetate  or  of  the  alkaline  lead  tartrate  solution  prepared  by  precip- 
itating a  soluble  lead  salt  with  tartaric  acid,  and,  redissolving  the 
precipitate  in  excess  of  caustic  alkali.  This  forms  a  more  bulky 
precipitate  which  carries  down  practically  the  whole  of  the  silver. 
The  precipitate  generally  settles  rapidly  and  is  easily  filtered ;  when 
much  zinc  is  present,  however,  there  may  be  some  difficulty,  and 
in  this  case  a  little  lime  may  be  added. 

After  washing  once  or  twice  with  dilute  sodium  sulphide  solution, 
the  filter  containing  the  precipitate  is  dried,  wrapped  in  lead  foil 
and  cupelled  at  a  moderate  temperature,  or  the  paper  may  be  al- 
lowed to  burn  slowly  on  a  scorifier,  and  the  ash  mixed  with  grain 
lead,  scorified  and  cupelled.  The  beads  of  silver  obtained  should  be 
free  from  gold,  and  this  will  be  the  case  if  the  sulphide  precipitate 
has  been  sufficiently  washed ;  but  in  any  case  it  is  better,  after  weigh- 
ing, to  dissolve  them  in  dilute  nitric  acid  and  deduct  the  weight  of 
any  gold  found. 

Gold  and  copper  are  not  precipitated  from  cyanide  solutions  by 
alkaline  sulphides;  in  weak  solutions  the  silver  precipitate  may 
carry  down  a  little  copper. 


METHOD  No.  2. 
Volumetric  Determination  with  Sodium  Sulphide. 

In  cases  where  the  cyanide  solution  contains  no  other  metals  ex- 
cept silver,  precipitable  by  alkaline  sulphides,  a  pretty  fair  approxi- 
mation may  be  made  by  running  in  standard  sodium  sulphide  to  a 
measured  volume  of  the  liquid  to  be  tested,  until  a  drop  taken  out 
on  a  glass  rod  gives  a  brownish  stain  with  a  drop  of  an  alkaline  lead 
tartrate  solution,  or  a  purple  color  with  sodium  nitroprusside. 
The  test  may  conveniently  be  made  by  placing  the  drops  side  by 
side  on  a  strip  of  white  filter  paper,  the  reaction  being  clearly  seen 
at  the  point  where  the  two  liquids  meet. 

The  sodium  sulphide  may  be  accurately  standardized  by  adding 
an  excess  of  a  solution  of  double  cyanide  of  silver  and  potassium, 
filtering,  and  titrating  the  liberated  cyanide  in  the  filtrate  in  the 
ordinary  way,  after  addition  of  potassium  iodide. 


=  0.3  gram 

=  0.828  gram  Ag. 


122  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

METHOD  No.  3. 
Precipitation  as  Sulphide  and  Conversion  into  Bromide. 

Allen  (Comm.  Org.  Anal.,  Vol.  III.,  pt.  3)  proceeds  as  fol- 
lows: A  definite  measure  of  the  liquid  is  boiled.  Sulphuretted 
hydrogen  is  passed  through  or  ammonium  sulphide  .added.  The 
silver  is  precipitated  as  Ag2S,  together  with  some  copper  and  zinc, 
if  these  metals  are  present.  The  precipitate  is  washed,  rinsed  into  a 
flask,  and  treated  with  excess  of  bromine  water.  If  sulphur  separates 
out,  more  bromine  must  be  added,  then  boiling  water.  The  silver 
bromide  is  then  washed,  dried,  fused,  and  weighed  as  AgBr.  AgBr 
X  0.5745  =  Ag. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  123 

CLASS  VII. 

BASE  METALS. 

General  Remarks. — The  base  metals  which  occur  in  ordinary 
cyanide  solutions  are  present  for  the  most  part  as  double  cyanides, 
consisting  of  more  or  less  stable  compounds  of  cyanogen  with  a  heavy 
metal  and  one  or  other  of  the  alkali  or  alkaline  earth  metals. 
Barium,  calcium,  etc.,  may  be  present  as  simple  cyanides.  In 
some  cases  (magnesium,  aluminum,  etc.)  the  metals  may  perhaps 
exist  as  hydrates,  double  chlorides,  or  other  compounds  not  con- 
taining cyanogen. 

The  metals  which  most  frequently  require  estimation  in  this  con- 
nection are  iron,  zinc  and  copper. 

The  estimation  of  iron  (as  ferro-  and  ferricyanides)  has  already 
been  described. 

When  metallic  zinc  in  any  form  is  used  as  a  precipitant  of  gold 
or  silver,  some  compound  of  zinc  is  necessarily  introduced  into  the 
solution,  and  its  estimation  sometimes  becomes  a  matter  of  im- 
portance. 

We  shall  here  discuss  only  the  estimation  of  zinc  and  copper. 
The  remaining  metals  are  determined  by  the  ordinary  methods  of 
analysis  in  the  residue  obtained  by  evaporation  of  the  solution,  after 
decomposing  cyanogen  compounds  as  described  below.  (Zinc, 
Method  No.  1.) 


SECTION  A. 
ESTIMATION  OF  ZINC. 

Zinc  is  generally  supposed  to  occur  in  cyanide  solution  as  a  double 
cyanide  of  the  type  of  K2ZnCy4,  though  there  is  some  evidence  to 
show  that  this  is  partially  dissociated  in  dilute  solutions,  which 
probably  contain  a  portion  of  the  zinc  as  the  simple  cyanide  ZnCy2. 
In  certain  cases  potassium  zincate  Zn(OK)2,  or  some  similar  com- 
pound, may  be  present. 


124  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

Zinc  may  be  determined  as  follows: 

1.  By  the  ordinary  methods  of  analysis  after  decomposition  of  the 
cyanogen  compounds. 

2.  By  precipitation  in  acidulated  solutions  with  ferrocyanide. 

3.  By  precipitation  in  alkaline  solutions  with  a  soluble  sulphide 
or  with  sulphuretted  hydrogen. 

4.  By  an  alkalimetric  method  based  on  the  reactions  between 
zinc  carbonate  and  ferrocyanide. 

METHOD  No.  1. 

Estimation  of  Zinc  after  Decomposition  of  Cyanide  by  Evaporation 
with  Acids. 

In  most  cases  the  solution  may  be  freed  from  all  cyanogen  com- 
pounds by  adding  to  a  measured  volume  (100  or  200  c.c.),  say,  5  c.c. 
of  concentrated  nitric  acid  and  5  c.c.  concentrated  sulphuric  acid, 
and  evaporating,  slowly  at  first,  finally  at  a  tolerably  high  tempera- 
ture, until  fumes  of  sulphuric  anhydride  (S03)  are  freely  given  off. 

The  residue  should  be  free  from  carbonaceous  matter,  and  should 
show  no  trace  of  any  blue  color  on  diluting  and  adding  hydro- 
chloric acid. 

If  this  treatment  fails  to  completely  decompose  the  cyanogen 
compounds,  it  will  be  necessary  to  evaporate  to  complete  dryness 
with  nitro-hydrochloric  acid,  adding  a  little  sulphuric  acid. 
Evaporation  with  volatile  acids  alone  would  be  liable  to  cause  loss 
of  zinc  through  volatilization  of  the  chloride.  In  some  cases  a  little 
potassium  chlorate  may  be  added  with  advantage. 

After  this  treatment,  add  2  or  3  c.c.  of  strong  hydrochloric  acid 
and  dilute  to  about  15  or  20  c.c.  Boil  till  all  soluble  matter  is  dis- 
solved, then  add  excess  of  ammonia  and  boil  again.  Filter  off  any 
precipitate  of  ferric  hydrate,  silica,  etc.  If  this  is  at  all  copious  it 
will  be  necessary  to  redissolve  the  precipitate  and  again  boil  with 
excess  of  ammonia;  filter  again,,  adding  the  second  filtrate  to  the 
first. 

If  copper  is  present,  acidulate  the  filtrate  with  hydrochloric  acid 
so  as  to  have  a  slight  excess  of  acid,  and  add  some  strips  of  lead ;  or 
acidulate  with  sulphuric  acid  and  add  sheet  aluminum ;  in  either  case 
boil  until  the  solution  becomes  colorless,  filter,  and  determine  zinc  in 
the  filtrate  by  any  of  the  ordinary  gravimetric  or  volumetric 
methods. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  125 

The  separation  of  iron  may  also  be  made  by  precipitation  as  basic 
acetate  in  cases  where  the  introduction  of  ammonium  salts  is  to  be 
avoided  (as  when  zinc  is  afterwards  to  be  precipitated  as  carbonate). 
This  may  be  done  as  follows :  After  adding  hydrochloric  acid,  neu- 
tralize with  sodium  hydrate  or  carbonate,  and  again  acidulate  very 
slightly  with  HC1.  Boil,  add  sodium  acetate  in  slight  excess,  boil 
again  to  precipitate  iron  and  alumina  as  basic  acetates.  This  pre- 
cipitate must  be  washed  several  times  by  decantation  with  hot  water 
before  transferring  to  the  filter,  and,  where  very  accurate  determina- 
tions are  required,  must  be  redissolved  and  reprecipitated.  The 
filtrate  should  be  slightly  acid,  and  perfectly  clear  and  colorless. 

Of  the  various  gravimetric  methods  for  estimating  zinc,  precipi- 
tation as  carbonate  and  weighing  as  oxide  is  probably  the  best. 
(See  Fresenius,  Quant.  Anal.,  7th  ed.,  Vol.  I.,  pp.  197,  433.)  Great 
care  is  necessary  to  wash  the  precipitate  thoroughly,  and,  in  igniting, 
to  avoid  reduction  of  the  oxide  to  metallic  zinc, 
ZnO  X  0.8026  =  Zn. 

The  volumetric  methods  in  general  use  are  based  on  one  of  two 
principles:  (a)  Precipitation  as  ferrocyanide;  (&)  Precipitation  as 
sulphide. 

(For  details  see  Sutton,  Volum.  Anal.,  8th  ed.) 


METHOD  No.  2. 

Estimation  of  Zinc  by  Precipitation  as   Ferrocyanide  in  Acid 
Solution. 

In  cases  where  the  solution  originally  contained  no  ferrocyanide, 
thiocyanate,  or  other  substance  capable  of  reducing  permanganate, 
and  where  metals  other  than  zinc  which  could  be  precipitated  by 
ferrocyanide  in  presence  of  free  sulphuric  acid  and  hydrocyanic  acid 
are  also  absent,  it  is  possible  to  estimate  the  zinc  as  follows  (Pro- 
ceedings Chem.  and  Met.  Soc.  South  Africa,  Vol.  I.,  p.  207)  : 

A  solution  of  potassium  ferrocyanide  is  accurately  standardized 
with  reference  to  a  solution  of  potassium  permanganate,  in  the  man- 
ner described  under  'Reducing  Agents.'  A  measured  volume  of 
the  liquid  to  be  tested  is  mixed  with  a  definite  volume  of  the  stand- 
ard ferrocyanide  solution,  diluted  to  about  200  c.c.,  or  until  not 
more  than  0.1  gram  ferrocyanide  is  present  per  100  c.c.  of  the 
liquid.  It  is  then  acidified  quite  strongly  with  sulphuric  acid  and 


126  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

titrated  with  permanganate.  The  equivalent  of  the  amount  of  per- 
manganate used  is  then  deducted  from  the  amount  of  ferrocyanide 
added,  the  difference  being  the  amount  of  ferrocyanide  used  in  pre- 
cipitating zinc. 

As  the  precipitated  ferrocyanide  of  zinc  interferes  with  the  titra- 
tion  with  permanganate,  the  excess  of  ferrocyanide  must  be  deter- 
mined by  making  up  the  solution  to  a  definite  volume,  allowing  to 
settle,  and  titrating  a  measured  quantity  of  the  clear  liquid.  The 
addition  of  a  little  pure  powdered  chalk,  before  acidulating  strongly 
with  sulphuric  acid,  was  found  to  promote  the  settlement  of  the 
zinc  ferrocyanide.  The  ferrocyanide  should  be  standardized  by 
means  of  a  zinc  cyanide  solution  containing  a  known  amount  of 
zinc. 

METHOD  No.  2  (MODIFIED). 

A.  F.  Crosse  (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol. 
III.,  p.  5,  May  10,  1902)  describes  the  following  modification  of 
the  above  process,  for  cases  in  which  ferrocyanides,  etc.,  are  present 
in  the  original  solution. 

The  ferrocyanide  is  determined  by  Bettel's  method  before  and 
after  the  precipitation  of  the  zinc,  the  difference  of  the  two  titra- 
tions  representing  the  ferrocyanide  converted  into  a  ferrocyanide  of 
zinc  and  precipitated. 

Crosse  gives  the  following  equation: 

K4FeCy6  +  2K2ZnCy4  +  4H2S04  = 

Zn2FeCy6  +  4K2S04  +  8HCy, 
or  1  part  K4FeCye  =  0.353  part  of  zinc. 

A.  Whitby  (Journal  Chem.  and  Met.  Soc.  South  Africa,  Vol. 
III.,  p.  15,  June  21,  1892)  states,  however,  that  potassic  zinc  ferro- 
cyanide is  generally  believed  to  be  formed,  the  reaction  consequently 
taking  this  form: 

2K4FeCy6  -f  3K2ZnCy4  -f  6H2S04  = 
K2Zn3(FeCy6)2  +  6K2S04  +  12HCy, 

which  materially  alters  the  calculation,  since,  if  the  latter  equation 
be  correct, 

1  part  K4FeCy6  =  0.265  part  of  zinc. 

It  is  necessary  to  add  about  0.1  per  cent  of  ferrocyanide  of  potas- 
sium to  the  cyanide  solution. 

The  method  is  carried  out  as  follows: 

(a)  20  c.c.  N/100  KMn04  is  placed  in  an  evaporating  dish  with 


CHEMISTRY    OF    CYANIDE    SOLUTION'S.  127 

some  dilute  sulphuric  acid,  and  the  cyanide  is  run  in  from  a  burette 
till  the  color  disappears.  The  result  is  calculated  into  the  equiva- 
lent of  K4FeCy6.  Let  A  =  K4FeCy6  used  per  100  c.c.  of  cyanide 
solution. 

(6)  A  second  portion  of  the  solution  is  taken  and  mixed  with  an 
equal  volume  of  dilute  sulphuric  acid  (5  per  cent  by  volume)  and 
the  ferrocyanide  of  zinc  filtered  off.  The  clear  solution  is  then  taken, 
poured  into  a  burette,  and  run  into  10  c.c.  of  N/100  KMn04  and 
the  result  calculated  into  its  equivalent  in  K4FeCy6.  Let  B  = 
K4FeCy6  per  100  c.c.  of  the  original  cyanide  solution.  A  —  B  is 
equivalent  to  the  amount  of  ferrocyanide  precipitated  by  the  zinc. 
1  c.c.  N/100  KMn04  =  0.0036849  gram  K4FeCy6. 

Thiocyanates  and  other  substances  which  reduce  permanganate  in 
acid  solution  do  not  interfere,  as  they  remain  the  same  in  both 
determinations. 

Of  course  this  method  is  only  applicable  for  solutions  containing 
chiefly  zinc,  and  with  mere  traces  of  copper,  etc.  Before  titrating 
it  is  advisable  to  warm  the  solution  slightly. 

Instead  of  using  permanganate,  it  is  probable  that  the  excess  of 
ferrocyanide  might  be  determined  by  titration  with  standard  copper 
sulphate  solution,  using  a  ferric  salt  as  external  indicator  as  in 
Bohlig's  method  for  ferrocyanides  (see  above) . 


METHOD  No.  3. 
Estimation  of  Zinc  by  Direct  Precipitation  as  Sulphide. 

Other  metals  precipitable  from  a  cyanide  solution  by  alkaline  sul- 
phides (e.g.,  silver  and  mercury)  must  be  absent. 

A  sodium  sulphide  solution  is  prepared  by  dividing  a  caustic  soda 
solution  into  two  equal  portions,  saturating  one  part  with  sul- 
phuretted hydrogen,  then  mixing  with  the  other  portion.  The  solu- 
tion may  be  standardized  against  a  zinc  sulphate  solution  containing, 
say,  44.15  grams  ZnS04-7H20  per  liter,  in  which  case  1  c.c.  =  0.01 
gram  Zn,  and  adjusted  until  it  corresponds  volume  for  volume  with 
the  zinc  solution. 

When  zine  is  precipitated  by  direct  addition  of  an  alkaline  sul- 
phide to  a  cyanide  solution,  the  precipitation  is  never  quite  com- 
plete, as  zinc  sulphide  is  not  absolutely  insoluble  in  alkaline  sul- 
phides. In  all  cases  the  precipitation  is  better  in  a  warm  solution, 


128  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

and  in  some  cases  it  appears  to  be  necessary  to  make  the  liquid 
strongly  alkaline  with  caustic  soda. 

The  finishing  point  may  be  found  by  various  external  indicators, 
among  which  may  be  mentioned: 

(a)  Alkaline  lead  tartrate.  (c)  Chloride  of  nickel. 

(6)   Sodium  nitroprusside.  (d)  Metallic  silver. 

Or  a  small  quantity  of  a  ferric  salt  may  be  added,  whicn  forms 
red  flakes  of  ferric  hydrate  in  the  alkaline  liquid.  On  running  in 
the  sodium  sulphide  from  a  burette,  a  white  precipitate  of  zinc  sul- 
phide is  formed  at  first,  but  when  the  zinc  is  completely  precipitated, 
any  excess  of  sodium  sulphide  causes  a  blackening  of  the  flakes  of 
ferric  hydrate,  which  thus  acts  as  an  internal  indicator. 

The  lead  tartrate  indicator  is  prepared  by  mixing  tartaric  acid 
and  caustic  soda,  then  adding  lead  acetate  till  a  permanent  precip- 
itate forms,  then  excess  of  soda  until  the  precipitate  redissolves. 
The  slightest  excess  of  sulphide  is  shown  by  placing  drops  of  the 
liquid  and  of  the  indicator  side  by  side  on  white  filter  paper,  when  a 
dark  stain  is  produced  where  the  two  liquids  meet. 

A  bright  silver  disc  may  also  be  used  for  determining  the  end 
point ;  a  drop  of  the  liquid  is  taken  out  on  a  glass  rod  and  placed 
for  10  or  20  seconds  on  the  silver  surface.  Zinc  sulphide  itself  has 
no  effect  on  the  silver,  but  any  excess  of  alkaline  sulphide  tarnishes 
the  silver  at  once. 

METHOD  No.  4. 

Estimation  of  Zinc  by  Precipitation  as  Sulphide,  and  Determining 
Excess  of  Sulphide. 

It  seems  to  be  pretty  generally  agreed  that  the  direct  determina- 
tion of  the  end-point  in  the  previous  method  is  uncertain  and  in- 
definite, at  least  in  ordinary  working  cyanide  solutions.  Hence 
several  methods  have  been  suggested  for  determining  the  excess  of 
sulphide,  after  adding  a  known  amount  of  standard  solution  more 
than  sufficient  for  complete  precipitation  of  the  zinc. 

The  following  will  be  here  noticed : 

(a)  Excess  of  sulphide  determined  by  sodium  nitroprusside  as 
in  Dr.  Loev/s  method. 

(6)  Excess  of  sulphide  determined  by  double  cyanide  of  silver 
and  potassium. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  129 

METHOD  No.  4  (a). 

Excess  of  Sulphide  Determined  by  Sodium  Nitroprusside. 

After  precipitating  the  warm  solution  with  sodium  sulphide  the 
liquid  is  filtered  and  the  precipitate  washed  free  from  sulphide. 
The  nitrate  is  then  titrated  by  the  colorimetric  method  described 
above.  (See  Sulphides.) 

METHOD  No.  4  (6). 

Excess  of  Sulphide  Determined  by  Double  Cyanide  of  Silver  and 
Potassium. 

This  process  has  been  described  by  the  writer  (Chemical  News, 
March  13,  1903).  It  depends  upon  the  fact  that  sulphides  de- 
compose the  double  cyanide  of  silver  and  potassium,  with  liberation 
of  an  equivalent  amount  of  cyanide,  thus : 

2KAgCy2  +  K2S  =  Ag2S  +  4KCy. 

(a)  After  precipitating  the  zinc  (best  in  a  warm,  strongly  al- 
kaline solution)  with  sodium  sulphide,  the  liquid  is  filtered  while 
still  hot,  and  washed  thoroughly  till  the  precipitate  is  free  from 
soluble  sulphides.     Another  method  is  to  cool  to  the  temperature  of 
the  room,  make  up  to  a  definite  volume  (say,  200  c.c.),  and  filter 
off  an  aliquot  part  (say,  100  c.c.). 

In  either  case  the  filtrate  is  mixed  with  a  moderate  excess  of  a 
double  cyanide  solution  [prepared  by  adding  silver  nitrate  to  a 
solution  of  pure  cyanide  (say,  0.5  per  cent  KCy)  until  a  permanent 
precipitate  forms,  allowing  to  stand  and  filtering] . 

After  thorough  agitation  with  this  liquid  the  precipitate  of  silver 
sulphide  is  allowed  to  settle,  filtered  and  washed.  5  c.c.  of  1  per  cent 
potassium  iodide  is  added  to  the  filtrate,  which  is  then  titrated  in  the 
ordinary  way  with  standard  silver  nitrate  solution. 

The  result  gives  the  cyanide  originally  present,  together  with 
that  liberated  by  the  excess  of  sulphide. 

(b)  Another  portion  of  the  liquid  is  titrated  direct  with  silver 
nitrate,   using  the   alkaline   iodide   indicator   to   determine   total 
cyanide.     This  result  deducted  from  the  first  gives  the  equivalent 
of  the  excess  of  sulphide. 


130  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

The  sodium  sulphide  solution  is  standardized  also  by  means  of 
the  double  silver  cyanide  solution, 

gram  Na2S. 


METHOD  No.  5. 

Precipitation  of  Zinc  as  Sulphide,  and  Determination  by  Decom- 
posing the  Precipitate  with  an  Oxidizing  Agent. 

Two  processes  based  on  this  principle  are  described  by  A.  F. 
Crosse,  in  which  the  sulphide  of  zinc  is  decomposed:  (i)  by  ferric 
sulphate;  (ii)  by  iodine. 

METHOD  No.  5  (a). 
Zinc  Sulphide  Decomposed  by  Ferric  Sulphate. 

The  following  method  (Journal  Chem.  and  Met.  Soc.  South 
Africa,  Vol.  III.,  p.  4,  May  10,  1902)  for  solutions  containing  not 
more  than  a  trace  of  copper  is  stated  to  be  very  exact  and  fairly 
quick  : 

"Take  300  c.c.  of  solution,  add  about  a  gram  of  KCy  and  the 
same  quantity  of  pure  caustic  potash  or  soda,  heat  nearly  to  boiling 
point,  and  then  add  a  slight  excess  of  sulphide  of  sodium  in  solu- 
tion. The  zinc  will  be  quickly  precipitated  as  sulphide,  and  should 
be  collected  on  a  filter  paper  and  washed  with  hot  water.  Then 
place  the  filter  paper  in  a  wide-mouthed  bottle  of  known  capacity, 
between  250  and  300  c.c.  This  bottle  must  be  provided  with  a 
well-fitting  india-rubber  bung  through  which  a  moderately  wide 
tube  is  inserted,  about  8  or  10  in.  long.  Then  fill  up  the  bottle 
with  a  weak  solution  of  pure  ferric  sulphate,  containing  5  per  cent 
to  7  per  cent  of  sulphuric  acid,  place  the  bottle  or  flask  in  a  bowl  of 
cold  water  and  raise  the  temperature  to  boiling  point.  The  reason 
for  the  glass  tube  will  be  apparent,  as  it  allows  for  the  expansion  of 
the  liquid. 

"The  zinc  sulphide  will  have  decomposed  [ZnS  -f  Fe2(S04)3  = 
2FeS04  +  ZnS04  -f-  S],  and  reduced  a  proportionate  amount  of 
ferric  sulphate  to  ferrous  sulphate. 

"When  nearly  cold,  filter  off  the  solution  through  a  dry  filter  pa- 
per, take  half  the  quantity  contained  in  the  bottle  and  titrate  with 
N/10  permanganate  :  1  c.c.  N/10  KMn04  =  0.00325  gram  Zn." 


CHEMISTRY    OF    CYANIDE    SOLUTION'S.  131 

The  results  in  presence  of  ferro  and  sulphocyanides  were  found 
by  Crosse  to  be  very  satisfactory. 

Owing  to  the  slight  solubility  of  zinc  sulphide  in  weak  cyanide 
solutions,  an  addition  of  one  milligram  of  zinc  per  100  c.c.  of  solu- 
tion taken  should  be  made  as  a  correction  to  the  result  obtained. 


METHOD  No.  5  (&). 
Zinc  Sulphide  Decomposed  by  Iodine. 

The  following  method  described  by  Crosse  (Journal  Chem.  and 
Met.  Soc.  South  Africa,  Vol.  III.,  p.  165)  is  quoted  from  Moor's 
Volumetric  Analysis,  p.  338. 

The  solutions  required  are: 

(a)  N/10  iodine. 

(-5)  N/10  thiosulphate. 

(c)  5  per  cent  sodium  sulphide. 

The  method  is  as  follows:  100  c.c.  of  the  working  solution  is 
heated  to  about  70°  C.,  and  an  excess  of  sodium  sulphide  added. 
The  mixture  is  allowed  to  settle,  filtered,  and  the  precipitate  of 
zinc  sulphide  well  washed  on  the  filter  with  hot  water ;  the  washing 
is  continued  until  the  wash- water  shows  no  trace  of  sulphide;  the 
filter  paper  is  then  transferred  to  a  small  150  c.c.  flask.  This  flask 
is  fitted  with  a  rubber  cork  having  two  holes;  through  one  passes 
a  drip  funnel  with  tap,  and  through  the  other  a  short  piece  of 
glass  tube,  terminating  in  a  small  length  of  rubber  tubing.  A 
pinch-cock  is  attached  to  the  latter.  Sufficient  N/10  iodine  is  then 
run  into  the  flask  to  leave  an  excess  of  not  more  than  5  c.c.  (30 
to  35  c.c.  N/10  iodine  is  usually  sufficient).  The  cork  is  then 
replaced,  and  about  100  c.c.  of  very  dilute  hydrochloric  acid  is  placed 
in  the  funnel.  The  tap  is  opened,  and  on  pressing  the  pinch-cock  air 
escapes  and  the  dilute  acid  takes  its  place.  The  taps  are  then 
closed,  and  the  whole  apparatus  shaken  to  thoroughly  break  up  the 
filter  paper.  After  a  few  minutes'  standing  the  contents  of  the 
flask  are  titrated  with  N/10  thiosulphate,  and  the  excess  of  iodine 
determined. 

If  x  =  No.  of  c.c.  N/10  iodine  taken, 

y  =  No.  of  c.c.  N/10  thiosulphate  required, 

(x  —  y)  X  0.00325  =  grams  Zn  per  cent. 


132  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

The  reactions  are  as  under: 

ZnS  +  2HC1  =  ZnCl2  -f  H2S. 

H2S  +  I2  =  2HI  -f  S. 
The  method  is  rapid  of  execution  and  capable  of  great  accuracy. 

METHOD  No.  6. 

Estimation  of  Zinc  by  Alkalimetric  Method,  Based  on  Precipitation 
as  Carbonate  and  Treatment  with  Potassium  Ferrocyanide. 

This  method,  due  to  L.  M.  Green  (Proceedings  Inst.  Min.  and 
Met.,  Vol.  X.,  pp.  29-37),  depends  on  the  fact  that  when  an  excess 
of  ferrocyanide  is  mixed  with  a  precipitate  of  basic  carbonate  of 
zinc,  there  is  a  liberation  of  alkali  in  proportion  to  the  zinc  present. 
It  is  carried  out  as  follows : 

(a)  50  c.c.  of  the  solution  to  be  tested  are  examined  for  total 
cyanide  by  Vielhaber's  method  (see  above),  to  determine  the 
amount  of  standard  silver  nitrate  required  to  give  a  permanent  red 
tinge  when  a  drop  of  neutral  potassium  chromate  has  been  added. 
If  necessary,  the  protective  alkali  must  be  neutralized  by  adding  the 
required  amount  of  N/10  nitric  acid  before  titrating  with  AgN03 
and  chromate. 

Let  n  =  No.  of  c.c.  of  AgN03  required. 

(&)  Another  50  c.c.  of  the  original  solution  are  now  taken,  n  c.c. 
of  silver  nitrate  added,  and  well  shaken.  A  few  drops  of  phenol 
phthalein  solution  (0.5  per  cent  in  alcohol)  are  now  added,  and  then 
N/10  sodium  carbonate,  until  a  distinct  pink  tinge  is  permanent. 
The  solution  is  now  very  cautiously  neutralized  by  the  addition  of 
N/10  nitric  acid  until  the  color  is  just  discharged.  The  liquid 
should  be  allowed  to  stand  for  a  few  moments  after  each  addition 
of  acid,  and  the  color  of  the  clear  liquid  observed  in  a  good  light 
against  a  white  background,  as  the  precipitate  may  remain  slightly 
tinged  after  the  liquid  is  really  neutral.  According  to  Green,  the 
zinc  is  thus  precipitated  as  basic  carbonate.  An  excess  of  ferro- 
cyanide is  now  added,  which  liberates  alkali  in  proportion  to  the 
amount  of  zinc  present.  This  alkali  is  now  determined  by  titrating 
the  resulting  pink  fluid  with  N/10  nitric  acid,  until  it  again  be- 
comes colorless  or  faintly  yellow. 

The  reactions,  so  far  as  the  zinc  double  cyanide  is  concerned, 
appear  to  be  as  follows,  according  to  the  explanations  given  in 
Green's  paper: 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  133 

(a)  On  adding  silver  nitrate  we  get  in  succession 

K2ZnCy4  +  AgN03  =  ZnCy2  +  KAgCy2  +  KN03, 
KAgCy2  +  AgN03  ^  2AgCy  +  KN03, 
ZnCy2  +  2AgN03  =  Zn(NOJ2  +  2AgCy. 
Or,  combining  these  in  one  equation : 

K2ZnCy4  +  4AgNO,  =  4AgCy  +  Zn(N03)2  +  2KNO,. 
(6)   On  adding  sodium  carbonate  to  neutralize 
2Zn(N03)2  +  3Na2C03  -f  2H20  = 

Zn(OH)2-ZnC03  +  2NaHC03  -f  4NaN03. 
[The  experimental  results  I  have  obtained  do  not  confirm  this 
equation,  but  point  rather  to  a  reaction  in  equivalent  proportions 
between  zinc  and  sodium  carbonate,  in  which  case  the  precipitate 
would  appear  to  consist  of  the  normal  carbonate,  thus : 
Zn(N03)2  +  Na2C03  =  ZnC03  +  2NaN03. 
The  matter,  however,  requires  further  investigation.] 

(c)  On  adding  ferrocyanide,  alkali  is  liberated 
Zn(OH)2-ZnC03  +  K4FeCy6  =  Zn2FeCy6  +2KOH  +  K2C03 
[Or  in  case  of  normal  zinc  carbonate 

2ZnC03  +  K4FeCye  =  Zn2FeCy6  +  2K2C03.] 

(d)  On  titration  with  standard  nitric  acid  (phenol  phthalein  in- 
dicator), 

2KOH  +  K2C03  +  3HN03  =  3KN03  +  KHC03  +  2H20,  or 
2K2C03  +  2HN03  =  2KHC03  +  2KN03. 

This  method  was  found  to  give  fair  comparative  results  for  vary- 
ing quantities  of  zinc.  It  requires  the  use  of  considerable  amounts 
of  silver  solution,  however,  and  the  neutralization  with  sodium  car- 
bonate and  N/10  acid  is  somewhat  tedious,  the  finishing  point  being 
rather  indefinite. 


SECTION  B. 
ESTIMATION  OF  COPPER. 

The  presence  of  copper  in  the  ore  under  treatment,  especially 
when  the  metal  occurs  as  carbonate,  has  a  very  injurious  effect,  giving 
rise  to  high  consumption  of  cyanide.  Moreover,  the  accumula- 
tion of  copper  in  the  working  solutions  beyond  a  certain  amount  in- 
terferes seriously  with  the  precipitation  of  the  gold.  Hence  in  some 


134  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

cases  it  becomes  a  matter  of  importance  to  estimate  the  quantity  of 
copper  in  the  solution. 

Detection  of  Copper. — The  presence  of  copper,  even  in  very  small 
amount,  may  be  detected  by  acidulating  the  cyanide  solution  with 
any  mineral  acid,  and  adding  a  few  drops  of  dilute  ferrocyanide 
solution,  which  gives  the  characteristic  reddish-brown  color. 

Ammonia,  of  course,  gives  no  blue  color  in  solutions  containing 
soluble  cyanides,  so  that  complete  decomposition  (e.g.,  by  boiling 
with  sulphuric  and  nitric  acids)  is  necessary  before  this  reagent  can 
be  applied  for  the  detection  of  copper. 

Methods  for  Estimating  Copper. — The  following  are  here  given : — 

(a)  Decomposition  of  the  solution  by  evaporation  with  acids, 
copper  being  afterwards  estimated  by  the  ordinary  analytical 
methods. 

(6)  By  direct  precipitation  from  cyanide  solution  as  cuprous 
cyanide,  by  addition  of  acid. 

GROUP  (a). — DECOMPOSITION  or  SOLUTION  BY  BOILING  OR  EVAPO- 
RATION WITH  ACIDS. 


METHOD  No.  1. 

Evaporation  with  Nitro-Hydrochloric  Acid,  Followed  by  Sulphuric 
Acid. 

This  process  has  already  been  described  under  zinc.  (See  above, 
Method  No.  1.)  In  the  majority  of  cases  it  appears  to  be  unneces- 
sary to  evaporate  to  complete  dryness;  it  is  sufficient  to  add  to 
100  c.c.  of  the  solution  5  c.c.  of  concentrated  nitric  and  5  c.c.  of 
concentrated  sulphuric  acid  and  boil  till  white  fumes  of  S03  are 
freely  given  off,  then  add  ammonia  in  slight  excess,  boil  again  and 
filter.  If  much  iron  is  present,  redissolve  the  precipitate  in  hydro- 
chloric acid  and  reprecipitate  with  ammonia,  filter  again,  adding 
the  second  filtrate  to  the  first. 

The  copper  may  now,  of  course,  be  estimated  by  any  of  the  ordi- 
nary gravimetric  or  volumetric  methods,  of  which  the  best  known 
are  the  'cyanide'  method  of  Parkes  and  the  'iodide'  method  of 
E.  0.  Brown,  both  of  which  are  extensively  used  with  various  modi- 
fications, and  are  described  in  all  text-books  on  analysis.  For  rapid 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  135 

estimations,  however,  the  simplest  plan  is  to  use  a  colorimetric 
method.  For  this  purpose  a  standard  solution  of  copper  nitrate  is 
prepared,  containing  0.1  per  cent  copper  and  about  1  per  cent  sul- 
phuric acid.  The  ammoniacal  copper  solution  from  the  cyanide  liq- 
uor, diluted  to  a  suitable  volume,  (according  to  the  amount  of  copper 
present)  say  10  c.c.,  for  every  milligram  of  Cu,  is  placed  in  a  clear 
glass  cylinder,  and  a  nearly  equal  volume  of  distilled  water  in  a  simi- 
lar cylinder.  To  the  latter  add  as  nearly  as  possible  the  same  amount 
of  ammonia  as  was  used  in  the  test,  and  run  in  the  copper  solution 
from  the  burette,  with  constant  shaking,  until  the  tint  in  the  two 
cylinders  is  the  same.  Every  c.c.  run  in  represents  0.001 
gram  Cu.  The  best  results  are  obtained  when  the  amount  of 
copper  is  between  5  and  15  milligrams.  With  practice,  the  amount 
of  copper  present  may  be  estimated  to  within  0.0002  gram.  Some 
difficulty  is  occasionally  experienced  in  getting  the  same  kind  of 
color  in  the  two  liquids;  this  appears  to  be  chiefly  owing  to  a  trace 
of  iron  passing  with  the  filtrate  into  the  ammoniacal  liquor,  to 
which  it  imparts  a  greenish  shade;  when  this  occurs,  no  accurate 
comparison  of  the  tints  is  possible. 

Where  the  quantity  of  copper  present  is  sufficient,  it  may  be  esti- 
mated quite  accurately  in  the  liquid  obtained  by  boiling  with  sul- 
phuric acid  till  fumes  are  given  off,  by  diluting  this  liquid  carefully 
to  about  100  c.c.,  adding  10  c.c.  of  concentrated  sulphuric  acid,  and 
one  or  more  sheets  of  aluminum,  boiling  for  10  minutes  and  treat- 
ing the  precipitated  copper  by  the  method  of  A.  H.  Low  (Journal 
Amer.  Chem.  Soc.,  XVIII.,  No.  5 ;  see  also  Button,  Volum.  Anal., 
8th  ed.,  p.  203). 

METHOD  No.  2. 
Treatment  with  Silver  Nitrate  and  Digestion  with  Acid. 

This  process  is  a  slight  modification  of  the  method  of  H.  Rose 
for  determining  total  cyanide  which  has  been  already  described. 

(a)  A  determination  is  made  of  the  amount  of  silver  nitrate  re- 
quired for  total  precipitation  of  cyanides  and  chlorides  with  chro- 
mate  indicator.  (See  Vielhaber's  method,  above.) 

(&)  Another  portion  of  the  original  liquid  is  now  taken  (say, 
10  to  50  c.c.),  and  the  amount  of  silver  nitrate  shown  to  be  neces- 
sary by  the  previous  test  for  complete  precipitation  of  cyanides, 
ferrocyanides,  chlorides,  etc.,  is  added  with  agitation,  then  5  c.c.  of 


136  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

concentrated  sulphuric  acid.  The  mixture  is  then  boiled  till  the 
residue  in  the  flask  is  perfectly  white,  or  has  only  a  slight  brownish 
tinge,  then  filtered,  and  the  flask  and  filter  washed  once  or  twice 
with  a  little  hot  water,  keeping  the  bulk  of  liquid  as  small  as  pos- 
sible. All  the  copper  will  now  be  in  solution  as  sulphate,  and  may 
be  estimated  as  in  the  previous  method. 

GROUP  (b). — PRECIPITATION  OF  COPPER  IN  CYANIDE  SOLUTION 
BY  DIRECT  ADDITION  OF  ACID. 

When  sulphuric  acid  is  added  in  slight  excess  to  a  solution  con- 
taining a  soluble  double  cyanide  of  copper,  a  white  precipitate, 
probably  cuprous  cyanide  (Cu2Cy2),  is  formed,  and  the  greater  part 
of  the  copper  is  thrown  down.  It  is  always  possible,  however,  to 
detect  copper  in  the  filtrate  by  the  ferrocyanide  test. 

The  following  method  devised  by  the  present  writer  (Journal 
Soc.  Chem.  Ind.,  Vol.  XIX.,  p.  14)  gives  accurate  results  under 
certain  conditions,  but  is  not  applicable  in  the  presence  of  zinc, 
silver  or  ferrocyanides. 

It  depends  upon  the  facts: 

1.  That  cuprous  cyanide  is  precipitated  from  a  solution  of  the 
double  cyanide  by  dilute  mineral  acids. 

2.  That  hydrocyanic  and  carbonic  acids  have  practically  no  ac- 
tion on  methyl  orange. 

3.  That  when  an  acid  is  added  gradually  to  a  mixed  solution 
containing  free  cyanide,  alkaline  hydrates  and  carbonates,  and  a 
double  cyanide  of  copper,  no  precipitation  of  copper  occurs  until 
the  whole  of  the  cyanides  and  alkaline  compounds  have  been  neu- 
tralized to  methyl  orange. 

The  test  is  made  by  adding  N/10  acid  in  slight  excess  to  a  meas- 
ured volume  of  the  liquid,  making  up  to  a  definite  volume,  filter- 
ing, and  titrating  an  aliquot  part  of  the  filtrate  with  N/10  sodium 
carbonate,  using  methyl  orange  as  indicator. 

The  initial  point  of  precipitation  with  ftf/10  acid  must  be  care- 
fully noted,  i.e.,  the  point  at  which  the  liquid  first  remains  perma- 
nently turbid  owing  to  precipitation  of  cuprous  cyanide.  From  the 
total  amount  of  acid  added  beyond  this  initial  point,  the  equivalent, 
for  the  entire  filtrate,  of  the  amount  of  N/10  carbonate  used  must 
be  deducted.  The  result  gives  the  amount  of  N"/10  acid  consumed 
in  precipitating  copper.  The  reaction  is  apparently 

K,2Cu2Cy4  -f-  2HN03  =  2KN03  -f  2HCy  +  Cu2Cy2. 


CHEMISTRY    OP    CYANIDE    SOLUTIONS.  137 

With  impure  solutions  a  difficulty  is  sometimes  experienced  in 
observing  the  initial  point;  also  the  titration  with  methyl  orange 
does  not  give  as  sharp  an  end  point  as  could  be  wished. 


SECTIONS  C,  V,  E. 
ESTIMATION  OF  IRON,  ALKALINE  EARTHS  AND  A  LIT  ALT  METALS. 

The  reader  is  referred  to  any  standard  work  for  the  details  of 
these  methods.,  as  they  present  no  special  difficulty. 


138  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

CLASS  VIII. 
SUSPENDED  MATTEK. 

General  Remarks. — The  nature  and  amount  of  the  solid  matter 
in  suspension  may  be  of  considerable  importance  under  certain  con- 
ditions. It  is  well  known  that  imperfect  precipitation  is  often 
caused  by  turbid  solutions,  and  in  some  cases  much  trouble  is  caused 
by  deposits  of  various  kinds  forming  on  the  surface  of  the  zinc 
shavings,  etc.  It  is  advisable  to  ascertain  by  occasional  tests  whether 
the  amount  of  suspended  matter  in  the  solution  is  increasing  or  not. 
The  insoluble  substances  carried  down  from  the  leaching  tanks  may 
be  very  various  in  character,  but  among  those  most  commonly  found 
may  be  mentioned:  (1)  Finely  divided  silica;  (2)  cyanides  and 
ferrocyanides  of  zinc;  (3)  ferric  hydrate,  alumina  and  magnesia; 
(4)  calcium  carbonate;  (5)  organic  matter  resulting  from  decay- 
ing vegetation  mixed  with  tailings. 

In  some  cases  the  suspended  matter  may  be  reduced  in  amount 
or  entirely  dissolved  by  increasing  the  cyanide  strength  or  the 
alkalinity  of  the  solution,  as,  for  example,  when  sparingly  soluble 
zinc  compounds  are  present. 

As  a  general  rule  it  will  be  sufficient  to  determine  the  total  amount 
of  suspended  matter,  but  occasionally  an  analysis  of  the  dried  resi- 
due may  be  required. 

We  shall  also  give  a  method  of  estimating  total  dissolved  solids. 


SECTION"  A. 
ESTIMATION  OF  TOTAL  SOLIDS  IN  SUSPENSION. 

For  this  purpose  a  measured  volume  (say  500  c.c.  or  1,000  c.c.) 
of  the  solution  is  passed  through  a  weighed  filter  paper,  and  the 
paper  weighed  again  after  washing  and  drying.  When  an  accurate 
determination  is  required  it  is  carried  out  as  follows : 

(a)  Preparation  of  Filters. — About  a  dozen  filter  papers  of  good 
quality  and  as  free  as  possible  from  ash  are  taken.  These  may  con- 
veniently be  about  9  c.m.  in  diameter.  The  filters,  several  together, 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  139 

are  folded  one  within  the  other  in  funnels,  and  washed  with  hot 
dilute  hydrochloric  acid  until  the  washings  show  no  trace  of  iron 
or  other  soluble  matter.  They  are  then  washed  with  hot  distilled 
water  until  no  trace  of  chlorine  can  be  detected  in  the  liquid  passing 
through,  on  testing  it  with  silver  nitrate.  The  papers  are  then 
spread  out  and  dried  thoroughly  on  a  water  bath. 

(&)  Determination  of  Ash. — Six  of  the  prepared  papers  may  be 
taken  for  this  purpose.  They  are  folded  together  into  a  small  packet 
and  wrapped  round  once  or  twice  with  one  end  of  a  piece  of  platinum 
wire.  Holding  the  wire  by  the  other  end,  the  papers  are  held  in  a 
small  gas  flame,  in  a  place  free  from  draughts,  until  completely 
burned.  The  ash  is  allowed  to  fall  into  a  clean  porcelain  crucible, 
which  has  previously  been  ignited  and  weighed;  this  is  then  again 
ignited  with  the  ash,  allowed  to  cool,  and  weighed.  The  quantity 
of  ash  found,  divided  by  6,  gives  the  average  weight  of  ash  for  one 
paper. 

(c)  Weighing  of  Paper  for  Collection  of  Suspended  Matter. — 
The  paper  to  be  used  for  this  purpose  is  dried  for  some  time  on  the 
water  bath,  placed  in  a  weighing  tube  and  weighed.    The  weighing 
tube  may  easily  be  made  from  a  couple  of  test  tubes,  one  of  which 
fits  tolerably  closely  within  the  other.    The  rim  of  the  smaller  tube 
is  cut  off,  and  the  edges  melted  smooth  in  a  small  gas  flame.    When 
the  paper  is  to  be  weighed  it  is  carefully  rolled  up  and  slipped 
inside  the  smaller  tube,  the  larger  tube  being  then  put  on  as  a 
cover.    After  weighing,  the  paper  is  returned  to  the  water  bath  and 
weighed  again  at  intervals  of  half  an  hour  until  the  weight  is  nearly 
or  quite   constant.     Two   consecutive   weighings   should   agree   at 
least  within  0.5  milligram.    The  weight  of  the  empty  tubes  is  then 
deducted.     The  tubes  and  paper  should  be  placed  in  a  desiccator  to 
cool  before  weighing. 

The  tubes,  during  drying,  may  conveniently  be  supported  over  the 
water  bath  on  a  sheet  of  paper  folded  in  grooves.  The  filter  is 
placed  almost  out  of  the  smaller  tube,  the  two  tubes  being  sepa- 
rated; they  can  then  be  shut  up  while  hot  by  pushing  the  tubes  to- 
gether on  the  grooved  paper. 

Filtration  will  generally  be  more  rapid  if  the  paper  be  folded  to 
form  a  ribbed  filter  before  drying. 

( d)  Filtration  of  Suspended  Matter. — When  the  quantity  of  sus- 
pended matter  is  very  large,  it  will  be  better  to  allow  a  sample  of 
the  liquid  to  remain  at  rest  for  several  hours  and  siphon  off  the 


140  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

clear  liquor,  which  may  then  be  passed  through  the  filter,  and  the 
sediment  added  after  the  whole  of  this  has  filtered  through.  In  other 
cases,  where  the  amount  of  sediment  is  not  excessive,  the  measured 
quantity  may  be  placed  in  a  narrow-necked  flask  and  inverted  over 
the  funnel  containing  the  weighed  filter  paper.  Finally,  the  residue 
adhering  to  the  sides  of  the  flask  is  carefully  rinsed  out  on  to  the 
funnel  with  hot  distilled  water,  and  the  filter  washed  until  the 
washings  show  no  indications  of  soluble  matter. 

(e)  Drying  and  Weighing  of  Suspended  Matter. — The  washed 
filter,  with  the  matter  upon  it,  is  now  dried  on  the  water  bath  with 
the  weighing  tubes  exactly  as  before.  The  weight  found,  less 
weight  of  tubes  and  paper  alone  (determined  above,  under  c),  gives 
the  total  solids  in  suspension,  dried  at  100°  C. 


SECTION  B. 
ESTIMATION  OF  VARIOUS  CONSTITUENTS  IN  SUSPENDED  MATTER. 

The  paper  with  its  contents,  after  drying  and  weighing  as  above, 
is  ignited  in  a  porcelain  crucible,  allowed  to  cool  and  weighed. 
The  amount  found,  less  ash  of  prepared  filter  paper,  gives  the  weight 
of  mineral  matter  in  suspension. 

The  loss  of  weight  on  ignition  represents  volatile  matter,  consist- 
ing of  organic  matter,  carbonic  acid,  combined  water,  etc. 

Examination  of  Ignited  Residue. — The  contents  of  the  crucible 
may  be  transferred  to  a  small  porcelain  dish  and  digested  for  some 
time  with  concentrated  hydrochloric  acid,  on  a  sand  bath,  finally 
evaporating  to  complete  dryness  and  igniting  gently.  The  residue  is 
then  examined  by  the  ordinary  methods  of  analysis  for  silica,  zinc, 
iron,  copper,  calcium,  magnesium,  alkalis,  etc. 

It  is  perhaps  advisable  to  determine  zinc,  and  some  other  metals 
which  might  volatilize  either  on  ignition  or  on  treatment  with  con- 
centrated hydrochloric  acid,  in  a  separate  portion  of  the  residue, 
dried  on  the  water  bath  without  ignition.  For  this  purpose  the 
residue  is  digested  with  nitro-hydrochloric  acid,  then  concentrated 
to  a  small  bulk,  a  few  drops  of  sulphuric  acid  added,  and  evapora- 
tion continued  until  S03  is  given  off.  Zinc,  iron,  copper,  etc.,  will 
then  be  present  as  sulphates. 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  141 

Determination  of  Calcium  and  Magnesium. — After  separation  of 
silica,  copper,  zinc,  iron,  aluminum,  etc.,  by  treatment  with  acids, 
H2S,  ammonia  and  ammonium  sulphide,  in  the  ordinary  way,  the 
filtrate  is  evaporated  with  nitric  acid  until  all  excess  of  sulphide 
is  oxidized,  filtering  again  if  necessary.  Hydrochloric  acid  is  then 
added,  heated  to  boiling,  then  ammonia  in  excess,  finally  oxalic  acid 
in  quantity  sufficient  to  precipitate  the  calcium,  leaving  an  excess 
of  ammonia.  The  liquid  is  well  stirred  and  allowed  to  stand  until 
it  has  settled  clear,  filtered,  washed  with  hot  water  till  free  from 
chlorides,  and  the  precipitate  strongly  ignited  and  weighed  as  CaO. 

The  filtrate  from  the  calcium  oxalate  precipitate  is  evaporated 
to  a  small  bulk,  a  few  drops  of  ammonia  added,  then  sodium  phos- 
phate, stirred  without  touching  the  sides  of  the  beaker  with  the  rod, 
allowed  to  settle,  filtered,  and  the  precipitate  washed  on  to  the  filter 
paper  with  a  little  of  the  filtrate;  finally  washed  with  25  per  cent 
ammonia  till  free  from  chlorides,  ignited,  and  weighed  as  mag- 
nesium pyrophosphate,  Mg2P207. 


SECTION  C. 
ESTIMATION  OF  TOTAL  SOLIDS  IN  SOLUTION". 

A  portion  of  the  filtrate  from  the  suspended  matter,  taken  before 
washing  the  latter  with  hot  water,  may  be  used  for  estimating  total 
solids  in  solution. 

Carefully  clean  and  ignite  a  small  porcelain  basin,  noting  its 
exact  weight  when  cool.  The  basin  must  be  of  such  a  size  that 
it  may  be  conveniently  placed  on  the  pan  of  an  analytical  balance, 
and  in  a  desiccator.  Evaporate  100  or  200  c.c.  of  the  solution  to 
dryness  in  this  basin,  on  water  bath ;  when  the  residue  appears  quite 
dry,  wipe  the  under  side  of  the  basin  with  a  cloth,  and  transfer 
to  desiccator  till  cool.  Weigh  approximately,  place  again  on  water 
bath.  Weigh  again  at  intervals  of  half  an  hour.  Before  removing 
the  dish  from  the  desiccator  it  is  advisable  to  place  the  weights 
previously  required  on  the  balance  pan,  so  as  to  be  able  to  take  the 
actual  weight  of  dish  and  contents  as  rapidly  as  possible.  The  dried 
residue  usually  absorbs  moisture  and  increases  in  weight  on  the 
balance  very  rapidly.  Eepeat  the  operation  till  the  difference  in 


142  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

weight  between  two  successive  weighings  is  less  than  1  milligram. 
About  three  hours  are  generally  required,  after  the  residue  is  appar- 
ently dry,  before  the  weight  is  constant. 

Weight  of  dish  and  contents,  less  weight  of  empty  dish,  gives  total 
solids  in  solution,  dried  at  100°  C. 

Note  color  and  appearance  of  residue. 

Total  Solids  on  Ignition. — In  some  cases  it  is  useful  to  heat  the 
basin  and  contents  gradually  to  redness,  after  having  obtained  a 
constant  weight  on  the  water  bath.  The  further  loss  of  weight  on 
ignition  gives  an  idea  of  the  amount  of  volatile  matter — cyanogen, 
C02  in  carbonates,  organic  matter  of  various  kinds,  ammonium  salts, 
water  of  crystallization,  etc.  Note  especially  if  any  charring  takes 
place,  if  fumes  are  evolved,  or  if  there  are  any  changes  of  color. 
Allow  to  cool  in  desiccator,  weigh,  ignite  again,  cool,  and  weigh  till 
weight  is  constant. 

The  residue  may  now  be  used  for  determination  of  silica,  iron, 
aluminum,  calcium,  magnesium,  potassium,  sodium  or  other  metals. 


CHEMISTEY    OF    CYANIDE    SOLUTIONS.  143 


AN  EXAMINATION  OF  VARIOUS  METHODS  FOR  THE 
ESTIMATION  OF  FERROCYANIDES. 

The  following  investigation  was  undertaken  with  a  view  to  ascer- 
taining which  of  the  numerous  published  processes  for  the  estima- 
tion of  ferrocyanides  would  be  most  suitable  for  use  in  dealing  with 
the  solutions  obtained  in  the  cyanide  treatment  of  ores.  None  of 
the  methods  examined  proved  to  be  entirely  satisfactory,  though  in 
some  cases  a  slight  modification  naturally  suggested  itself,  which 
gave  decidedly  better  results  than  the  original  form  of  the  process. 
The  following  is  a  short  summary  of  methods  examined,  which  were 
selected  as  being  the  most  promising : 

SUMMARY  OF  METHODS  EXAMINED. 

(1)  Titration  of  iron,  after  decomposition  of  the  cyanogen  com- 
pounds with  acids. 

(a)  Without  precipitating  the  iron  as  hydrate. 
(&)  By  precipitating  with  ammonia  and  determining  iron  in 
the  precipitate. 

(2)  Inverse  titration  with  permanganate. 

(a)  Running  solution  to  be  examined  into  a  measured  volume 
of  standard  permanganate  (Bettel's  method). 

(&)  Determining  excess  of  permanganate  by  means  of  potas- 
sium iodide  and  thiosulphate. 

(3)  Precipitation  as  Prussian  blue,  treating  precipitate  with  al- 
kali, filtering  and  titrating  the  acidulated  filtrate. 

(a)  With  permanganate  (Erlenmeyer's  method). 
(&)  With  copper  sulphate  (Knublauch's  method). 

(4)  Direct  titration  with  standard  copper  sulphate  and  ferric  in- 
dicator (Bohlig's  method). 

(5)  Conversion  into  ferrocyanide  and  titration  of  the  latter  with 
copper  sulphate  and  ferrous  indicator. 

(a)  Converted  by  bleaching  powder  (Hurter's  method). 
(&)  Converted  by  permanganate. 

(6)  Titration  with  standard  zinc  sulphate  (Miiller's  method). 

(7)  Green's  allcalimetric  method. 


144  CHEMISTRY    OP    CYANIDE    SOLUTIONS. 

METHOD  No.  1. 
Titration  of  Iron. 

This  method  is  based  on  the  assumption  that  all  the  iron  in  the 
solution  is  present  as  ferrocyanide.  As  generally  carried  out,  the 
solution  is  evaporated  several  times  to  dryness  with  strong  acids,  in 
order  to  ensure  the  complete  decomposition  of  the  cyanogen  com- 
pounds. In  most  cases,  however,  this  was  found  to  be  unnecessary, 
and  complete  removal  of  cyanogen  could  be  effected  as  follows : 

Method  No.  1  (a) .  Without  Precipitation  of  Iron  as  Hydrate. — 
A  measured  volume  of  a  solution  containing  the  usual  impurities 
found  in  cyanide  liquors  (K2ZnCy4,  KCyS),  together  with  a  known 
volume  of  N/100  ferrocyanide  (4.22  grams  K4FeCy6-3H20  per 
liter),  was  evaporated  with  addition  of  5  c.c.  concentrated  nitric 
acid  and  10  c.c.  sulphuric  acid  (25  per  cent  per  volume  of  strong 
acid),  until  dense  white  fumes  of  sulphuric  acid  were  evolved.  When 
cool,  25  c.c.  water  and  10  c.c.  of  the  above  25  per  cent  sulphuric 
acid  were  added,  then  one  gram  of  clean  zinc,  and  the  mixture  heated 
gently  until  the  zinc  had  completely  dissolved.  The  flask  was  then 
cooled,  50  c.c.  of  boiled  distilled  water  added,  and  the  solution  ti- 
trated immediately  with  N/100  permanganate. 

The  sulphuric  acid  used  was  prepared  by  dropping  permanganate 
solution  into  the  diluted  acid,  after  cooling,  until  a  very  faint  pink 
tint  remained  permanent. 

The  zinc  used  left  a  small  quantity  of  black  residue  on  dissolving 
in  acid.  It  was  not  found  advisable  to  filter  this  off,  but  a  correction 
was  made  by  carrying  out  a  blank  experiment,  using  the  same  weight 
of  zinc  and  of  other  ingredients,  but  omitting  ferrocyanides.  The 
amount  of  permanganate  finally  required  in  titrating  the  blank  test 
was  deducted  from  the  numbers  obtained  in  the  other  determinations. 
Usually,  however,  it  was  found  that  a  larger  correction  than  that 
shown  by  the  blank  test  was  necessary,  in  order  to  give  numbers  ap- 
proximately proportional  to  the  ferrocyanide  taken. 

For  convenience  in  comparison  the  results  of  each  determination 
have  been  calculated  so  as  to  show  the  percentage  variation  from  the 
mean  result.  (See  last  column.) 


CHEMISTRY    OF    CYANIDE    SOLUTION'S. 

Table  No.  1.    Results  of  Method  No.  1  (a). 

Model  Solution  A      r*Qy  0.19296, 

used  in  tests. 


145 


K2ZnCy4    0.076 %, 
KCyS         0.03  %. 


Permanganate        f  1  c.c.  =  0.000536  gram  Fe. 
(standard  solution)   \  =  0.04042     gram  K4FeCy6-3H20. 


Solutions  Tested. 

(a) 
Standard 
permanganate 
required. 

c.c. 

(b) 
Corrected 
volume  of 
permanganate. 
b  =  a—  4.1 

c.c. 

(c) 
Permanganate 
per  100  c.c. 
of  N/100  ferro- 
cyanide. 

c.c. 

(d) 
Percentage 
results. 
94.3  =  100 

N/100 

Ferro- 
cyanide 
c.c. 

Solution 
A 
taken, 
c.c. 

10 
20 
25 
30 
40 
50 

50 
50 
50 
50 
50 
50 

13.45 
22.7 
28.25 
32.25 
42.25 
50.9 

9.35 
18.6 
24.15 
28.15 
38.15 
46.8 

93.5 
93. 
96.6 
93.8 
95.4 
93.6 

99.2 
98.6 
102.4 
99.5 
101.2 
99.3 

The  results  obtained  in  a  similar  series  of  tests  with  pure  ferro- 
cyanide  (without  adding  KCy,  K2ZnCy4  and  KCyS)  showed  about 
the  same  degree  of  divergence.  We  may  conclude,  therefore,  that 
this  method,  with  the  proper  corrections,  would  give  results  within 
about  ±  2.5  per  cent  of  the  actual  amount  of  ferrocyanide  present. 

Method  No.  1  (6).  Precipitating  Iron  with  Ammonia. — A 
measured  volume  of  the  solution  to  be  tested,  containing  a  known 
amount  of  ferrocyanide,  was  evaporated  with  5  c.c.  concentrated 
nitric  acid  and  10  c.c.  special  25  per  cent  sulphuric  acid — prepared 
as  described  under  Method  No.  1  (a) — until  white  fumes  were  freely 
evolved,  and  the  cyanogen  compounds  appeared  to  be  completely 
decomposed.  Afi'er  cooling  somewhat,  5  c.c.  of  concentrated  hydro- 
chloric acid  were  added,  heated  to  boiling,  then  25  c.c.  of  water, 
and  ammonia  in  slight  excess.  After  again  boiling,  the  precipitate 
of  ferric  hydrate  was  filtered  off  and  washed  with  hot  water  till 
free  from  chlorides.  It  was  then  dissolved  from  the  filter  paper 
into  a  clean  flask  by  means  of  10  c.c.  of  special  sulphuric  acid,  the 
paper  washed  with  hot  water  till  free  from  iron,  collecting  the  wash- 
ings in  the  same  flask,  and  reduced  by  dissolving  1  gram  of  clean 
zinc  in  the  liquid.  When  the  zinc  had  completely  dissolved,  the 
flask  was  cooled,  50  c.c.  of  boiled  distilled  water  added,  and  the 
solution  titrated  immediately  with  permanganate. 


146 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


As  in  the  previous  method,  a  correction  is  necessary,  as  not  only 
the  zinc,  but  also  the  filter  papers  contain  a  small  amount  of  iron 
or  other  reducing  agent.  A  blank  experiment  is  conducted  exactly 
as  in  the  actual  determinations,  but  omitting  ferrocyanide. 

This  method  has  the  advantage  that  zinc,  copper,  etc.,  may  be 
determined  in  the  filtrate  from  the  ferric  hydrate  precipitate. 


Table  No.  2.    Results  of  Method  No.  1  (&) 
Model  Solution  A 


KCy  0.1929&, 
K2ZnCy4  0.076 %, 
KCyS  0.03  %. 


Standard 
Permanganate 


{1  c.c.  =  0.000536  gram  Fe, 
=  0.1 


0.04042  gram  K4FeCye-3H20. 


Solutions  Tested. 

(a) 
Standard 

<&) 
Corrected 

(o) 
Permanganate 

(<*) 
Percentage 

N/100 
Ferro- 

Solution 
A 

permanganate 
required. 

volume  of 
permanganate, 
b  =  a  —  1.5 

per  100  c.c. 
of  N/100  ferro- 
cyanide. 

results. 
104  =  100 

cyanide 

taken. 

taken. 

c.c. 

c.c. 

c.c. 

c.c. 

10 

50 

11.9 

10.4 

104 

100 

20 

50 

22.6 

21.1 

105.5 

101.4 

25 
30 

I 

27.7 
32.7 

26.2 
31.2 

104.8 
104 

100.8 
100 

40 

50 

43.15 

41.65 

104.1 

100.1 

50 

50 

52.9 

51.4 

102.8 

98.8 

50 

53.75 

52.25 

104.5 

100.5 

The  results  obtained  by  this  method  thus  appear  to  vary  within 
±  1.5  per  cent  of  the  theoretical  value. 

METHOD  No.  2. 
Inverse  Titration  with  Permanganate. 

Direct  titration  with  permanganate  as  originally  described  by 
de  Haen  is,  of  course,  inapplicable  in  presence  of  zinc  compounds 
and  thiocyanates.  The  modification  described  by  Bettel  involves 
two  separate  determinations: 

(t)  The  total  reducing  power  obtained  by  running  the  solu- 
tion to  be  tested  into  a  measured  volume  of  permanganate,  to  which 
sulphuric  acid  has  been  added,  until  the  color  is  discharged. 

(H)  The  reducing  power,  exclusive  of  ferrocyanide,  which  is 


CHEMISTRY    OF    CYANIDE    SOLUTIONS.  147 

obtained  by  precipitating  with  an  acid  ferric  salt,  filtering,  and 
titrating  the  filtrate. 

The  difference  of  these  two  determinations  is  supposed  to  give 
the  reducing  power  of  the  ferrocyanide. 

The  results  obtained  when  the  test  was  made  as  directed  by  Bettel 
were  not  at  all  satisfactory,  the  end-point  in  determination  (i)  was 
by  no  means  sharp,  and  the  complete  washing  of  the  precipitate 
in  (M)  was  a  lengthy  and  troublesome  operation.  Owing  to  the 
high  reducing  power  of  thiocyanates  as  compared  with  ferro- 
cyanides,  the  thorough  washing  of  the  precipitate  in  this 
test  is  most  essential.  As,  however,  the  estimation  of  total  reducing 
power  may  in  some  cases  be  a  matter  of  importance,  some  tests  were 
made  on  the  first  of  Bettel's  determinations  (Proceedings  Chem.  and 
Met.  Soc.,  S.  Africa,  Vol.  I.,  p.  166). 

Method  No.  2  (a).  Inverse  Titration  with  Acidulated  Perman- 
ganate.— A  measured  quantity  of  permanganate  solution  (approx- 
imately N/100)  was  placed  in  a  white  porcelain  basin  with  addi- 
tion of  from  10  to  30  c.c.  of  25  per  cent  sulphuric  acid  (prepared 
as  in  the  previous  tests,  by  addition  of  permanganate  until  a  faint 
pink  tint  remained  permanent).  The  solution  to  be  tested,  con- 
taining known  amounts  of  ferrocyanide,  together  with  the  usual  im- 
purities, was  then  run  from  a  burette  into  the  acidulated  perman- 
ganate until  the  pink  color  changed  to  a  faint  yellow.  When  much 
permanganate  was  used,  the  addition  of  the  solution  caused  the 
formation  of  a  brown  precipitate,  in  which  case  no  definite  finish- 
ing point  could  be  observed.  This  was  remedied  to  some  extent  by 
increasing  the  amount  of  acid. 


Table  No.  3.    Results  of  Method  No.  2  (a). 
Model  Solution  B 


KCy  0.15%, 

K2ZnCy4  0.196, 

KCyS  0.075fo, 

K4FeCya-3H20  0.053 96. 


Standard  Permanganate    J 

[=  0.316  gram  per  liter. 


148 


CHEMISTRY   OF   CYANIDE   SOLUTIONS. 


N/100 
Permanganate 
taken. 

c.c. 

Special 
sulphuric   acid 
25*  by  vol. 

c.c. 

Model  Solution 
B  required. 

C.C. 

N  /100  KMnO« 
consumed 
per  100  c.c.  B. 

c.c. 

Reducing  power 

554=100 

80 
40 
50 
100 
100 

10 
10 
10 
20 
80 

5.5 
7.1 
9.2 
17.9 
17.8 

545 
563 
543 
559 

562 

98.4 
101.6 
98 
100.9 
101.4 

Similar  sets  of  tests  were  made  with  various  different  mixtures, 
giving  in  most  cases  results  varying  within  ±  2  per  cent  of  the  theo- 
retical values. 

The  N/100  permanganate  solution  does  not  retain  its  strength 
for  any  length  of  time,  and  requires  to  be  standardized  at  frequent 
intervals  (on  iron  or  pure  ferrocyanide). 

Method  No.  2  (6).  Determining  Excess  of  Permanganate  with 
Potassium  Iodide  and  N/100  Thiosulphate.  —  As  before,  the  solu- 
tion to  be  tested  was  run  into  a  measured  volume  of  approximately 
N/100  permanganate,  acidulated  with  from  10  to  30  c.c.  of 
"special"  sulphuric  acid,  but  the  permanganate  was  allowed  to  re- 
main in  excess.  About  2  c.c.  of  1  per  cent  potassium  iodide  solution 
were  then  added,  and  the  liberated  iodine  titrated  by  means  of 
N/100  sodium  thiosulphate  (2.48  grams  Na2S203-5H20  per  liter). 
The  amount  of  thiosulphate  required  is  then  deducted  from  the 
equivalent  amount  of  permanganate  originally  taken,  the  difference 
giving  the  amount  of  permanganate  reduced  by  the  solution  tested. 

Table  No.  4.    Results  of  Method  No.  2  (&). 
f    KCy  0.15%, 


K4FeCye-3H20 


0.152%. 


Perman- 
ganate 
taken. 

c.c. 

Equivalent 
to  N/100 
permang. 

c.c. 

Model 
solution 
C 
taken. 

c.c. 

N/100  thio- 
sulphate 
required. 

C.C. 

N/100 
KMnO4 
consumed. 

c.c. 

N/100 
KMn04 
consumed 
per  100  c.c. 
ofC. 
c.c. 

Reducing 
power 
per  cent. 
434=100 

50 
60 
50 
60 
60 
50 

45.1 
45.1 
45.1 
45.1 
45.1 
45.1 

4 
5 
6 
7 
8 
9 

27.45 
28.4 
19.35 
14.7 
10.4 
8.2 

17.65 
21.70 
25.75 
80.40 
84.70 
38.90 

441 

434 
429 
434 
484 
432 

101.6 
100 
98.9 
100 
100 
99.5 

CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


149 


The  results  obtained  by  this  method  generally  agreed  with  about 
3  per  cent  of  the  theoretical  numbers,  in  some  cases  the  agreement 
was  much  closer;  in  one  set  the  divergence  amounted  only  to  0.5 
per  cent.  It  is  essential  that  the  permanganate  solution  should  be 
standardized  on  the  N/100  thiosulphate  immediately  before  or  after 
making  the  required  determinations.  The  finishing  point  is  ascer- 
tained by  adding  a  few  drops  of  starch  solution,  either  freshly  pre- 
pared or  preserved  by  the  addition  of  a  little  caustic  soda.  The  blue 
color  generally  shows  some  tendency  to  return  on  standing,  but  the 
reaction  may  be  considered  complete  when  the  solution  remains 
quite  colorless  for  a  minute. 

The  amount  of  permanganate  apparently  consumed  in  oxidizing 
the  various  oxidizable  ingredients  in  the  solution  was  considerably 
less  in  this  method  than  in  Method  No.  2  (a)  (Bettel's  form),  as 
shown  in  the  following  comparative  results  : 


Table  No.  5. 


Model  Solution  D 


0.15%, 


KCy 

K2ZnCy4 

KCyS         .       0.075%, 

K4FeCy6-3H20  0.158%. 


Method  No.  2  (o). 

Method  No.  2   (b) 

Volume  of 

permanganate 
taken. 

Model 
solution  D. 

KMnO«  per 
100  c.c.  of 

Model 
solution  D. 

KMnO.  per 
100  c.c.of  D. 

required. 

D 

required. 

c.c. 

c.c. 

c.c. 

c.c. 

40 

7.5 

533 

8.6 

464 

50 

9.2 

543 

10.5 

478 

60 

11.7 

513 

13.2 

455 

METHOD  No.  3. 
Precipitation  as  Prussian  Blue. 

This  operation  is  involved  as  a  necessary  step  in  Bettel's  method 
(described  above)  and  in  Erlenmeyer's  and  Knublauch's  methods. 
In  Bettel's  method  the  precipitate  of  Prussian  blue  is  not  dissolved 
again,  but  it  must  be  washed  and  the  filtrate  titrated  with  perman- 
ganate. In  the  other  methods  the  precipitate  is  treated  with  alkali, 
and  the  resulting  ferrocyanide  titrated,  after  acidulating,  with  per- 
manganate or  copper  sulphate  respectively. 


150 


CHEMISTRY    OP    CYANIDE    SOLUTIONS. 


Method  No.  3  (a)  (Erlenmeyer's) .  Precipitation  as  Prussian 
Blue,  Conversion  into  a  Soluble  Ferrocyanide  and  Titration  of  the 
Latter  with  Permanganate. — The  mixture  containing  known  quan- 
tities of  KCy,  KCyS,  K2ZnCy4  and  K4FeCy6  was  heated  to  boiling 
and  poured  into  10  c.c.  of  special  ferric  chloride  solution,  also  boil- 
ing. This  special  solution  contained  about  6  per  cent  of  ferric 
chloride  and  20  per  cent  hydrochloric  acid. 

The  mixture  was  stirred  and  allowed  to  settle,  then  washed  by 
decantation,  and  finally  collected  on  a  filter  and  washed  thoroughly 
with  boiling  water  till  the  washings  were  free  from  chlorine  and 
thiocyanates.  The  precipitate  was  then  washed  off  the  filter  paper 
as  far  as  possible  into  a  clean  flask,  and  10  c.c.  of  5  per  cent  caustic 
soda  passed  through  the  funnel  into  the  same  flask.  This  was  then 
boiled  for  some  time  until  the  Prussian  blue  appeared  to  be  com- 
pletely decomposed,  and  filtered  through  the  same  filters.  The 
ferric  hydrate  precipitate  was  then  washed  until  free  from  ferro- 
cyanide,  and  the  filtrate  acidulated  by  the  addition  of  10  c.c.  25 
per  cent  sulphuric  acid.  The  resulting  liquid  was  titrated  with 
N/100  permanganate. 

The  method  is  long  and  tedious,  as  great  care  must  be  taken  in 
the  washing  of  both  precipitates,  which  must  be  done  with  very 
hot  water. 

Attempts  made  to  determine  the  ferrocyanide  by  redissolving  the 
ferric  hydrate  precipitate  and  treating  as  in  Method  No.  1  (&)  did 
not  give  satisfactory  results. 

Table  No.  6.     Results  of  Method  No.  3   (a). 
KCy         0.15  %, 


Model  Solution 
Ferrocyanide  solution 


tion  E    J 


K2ZnCy4 
KCyS       0.05  %, 

N/100  =  OA22%. 


N/100  ferro- 
cyanide 
taken. 

Model 
solution  E 
added. 

Permanganate 
required 

Permanganate 
N/100  ferrocy- 
anide. 

Percentage 
results 
102.8  -  100. 

per  100  c.c.  of 

C.C. 

c.c. 

c.c. 

10 

60 

10.1 

101 

98.2 

20 
25 

60 
50 

20.95 
25.3 

104.7 
101.2 

101.9 

98.4 

30 

50 

31.1 

103.6 

100.9 

40 

50 

39.9 

99.7 

97  0 

50 

50 

53.2 

106.4 

103.5 

CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


151 


These  results  show  a  variation  from  the  mean  value  of  ±  3.5  per 
cent. 

As  the  method  is  scarcely,  if  at  all,  more  rapid  than  Method 
No.  1,  it  does  not  appear  to  present  any  special  advantages. 

Method  No.  3  (&)  (Knublauch's) .  Precipitation  as  Prussian 
Blue,  Conversion  into  a  Soluble  Ferrocyamde,  and  Titration  of  the 
Latter  with  Copper  Sulphate  and  Ferric  Indicator. — The  operations 
are  practically  the  same  as  in  Erlenmeyer's  method,  but  after  acidu- 
lating the  final  filtrate  with  sulphuric  acid,  the  liquid  is  titrated  by 
running  in  copper  sulphate  (about  1  per  cent  CuS04-5H20)  until 
a  drop  of  the  liquid  no  longer  gives  a  blue  color  with  a  drop  of 
dilute  ferric  chloride,  when  the  two  drops  are  allowed  to  run  into 
one  another  on  white  filter  paper.  Only  a  few  tests  were  made  by 
this  method,  which  presents  all  the  drawbacks  of  Erlenmeyer's 
method,  with  the  additional  disadvantage  that  the  final  titration 
requires  an  external  indicator. 

Table  No.  7.    Results  of  Method  No.  3  (&). 


Solutions  Tested.* 

Special 
6% 
ferric 
chloride 
taken 
C.C. 

CuSO* 
(about 
1%) 
required. 

c.c. 

CuSO« 
per 

gram, 
ferrocy- 

anide. 

C.C. 

Percent, 
results 
74.1  =  100 

Ferro- 
cyanide. 
gram. 

KCy 
gram. 

KCyS 
gram. 

K,ZnCy« 
gram. 

0.105 
0.211 

0.093 
0.037 

0.03 
0.03 

0.02 
0.01 

10 
10 

7.6 
16.0 

72.4 
75.8 

.   97.7 
102.3 

c.c.  of  liquid  In  each 


METHOD  No.  4. 
Direct  Titration  with   Copper  Sulphate. 

This  method,  due  to  E.  Bohlig,  is  described  by  Fresenius  (Quant. 
Anal.,  7th  ed.,  Vol.  I.,  p.  380).  It  was  found  to  give  moderately 
good  results  in  absence  of  zinc,  but  the  presence  of  this  element 
entirely  vitiates  the  test. 

Some  of  the  results  obtained  in  absence  of  zinc  are  given  below. 
A  modification  of  the  method  was  also  tried  in  which  the  zinc  was 
removed  by  preliminary  treatment  with  sodium  sulphide. 

Method  No.  4  (a).    Direct  Titration  with  Copper  Sulphate  in 


152 


CHEMISTRY   OP   CYANIDE   SOLUTIONS. 


Absence  of  Zinc. — A  measured  volume  of  the  solution  was  acidu- 
lated with  sulphuric  acid,  and  titrated  with  1  per  cent  solution  of 
copper  sulphate,  until  a  drop  of  the  liquid  taken  out  on  a  glass 
rod  and  placed  on  filter  paper  beside  a  spot  of  very  dilute  ferric 
chloride  no  longer  showed  any  blue  coloration.  The  ordinary  white 
filter  paper  was  found  to  contain  sufficient  iron  to  give  a  distinct 
blue  color  as  long  as  a  considerable  excess  of  ferrocyanide  was 
present,  but  the  exact  finishing  point  was  determined  by  means  of 
the  ferric  chloride  solution. 

Table  No.  8.    Results  of  Method  No.  4.  (a). 


Contents  of  Solution  Tested. 

Volume 
of  25* 
H,SO« 
added. 

c.c. 

CuSO4 
required. 

c.c. 

CuSO4 
per 
gram  of 
ferrocy. 

c.c. 

Percentage 
results. 
81.1  =  100 

Ferro- 
cyanide. 
gram. 

Cyanide 
KCy 
gram. 

KCyS 
gram. 

0.811 
0.211 
0.211 
0.211 
0.211 
0.211 
0.042 
0.105 
0.211 

0.087 
0.087 
0.175 

0.069 
0.027 
0.069 

.06 
.06 
.03 
.03 
.03 
.03 

4 
4 
4 
4 
4 
4 
4 
4 
4 

16.4 
17.5 
17.0 
17.5 
17.0 
17.0 
3.5 
8.5 
17.0 

77.7 
82.9 
80.6 
82.9 
80.6 
80.6 
83.3 
81.0 
80.6 

95.2 

102.2 
99.4 
102.2 
99.4 
99.4 
102.7 
99.9 
99.4 

These  results,  excluding  the  first,  show  values  varying  within 
±  3  per  cent  of  the  theoretical  amount.  The  result  does  not  appear 
to  be  affected  by  variation  within  moderate  limits  in  the  amount  of 
cyanide  and  thiocyanate  present. 

In  presence  of  zinc  the  results  were  always  too  low,  probably 
owing  to  the  conversion  of  a  part  of  the  ferrocyanide  in  the  acidu- 
lated liquid  into  ferrocyanide  of  zinc.  The  following  modification 
was  therefore  tried: 

Method  No.  4  (&).  Treatment  with  Sodium  Sulphide  to  Remove 
Zinc,  and  Titration  of  the  Filtrate  with  Copper  Sulphate. — A 
measured  volume  of  the  solution  to  be  tested  was  mixed  with  strong 
alkali  and  an  excess  of  sodium  sulphide  solution,  well  agitated,  fil- 
tered, and  the  precipitate  of  zinc  sulphide  washed  until  apparently 
free  from  ferrocyanide.  The  filtrate  was  then  agitated  with  lead 
carbonate  to  remove  excess  of  sulphide,  filtered  again,  washed, 
and  the  filtrate  acidulated  with  sulphuric  acid,  and  titrated  as  above 
with  1  per  cent  copper  sulphate,  and  dilute  ferric  chloride  as  ex- 
ternal indicator. 

The  precipitate  of  lead  sulphide  appeared  to  carry  down  a  little 


CHEMISTRY   OF    CYANIDE   SOLUTIONS. 


153 


ferrocyanide  which  was  not  easily  washed  out;  also  some  lead  dis- 
solved in  the  strongly  alkaline  liquid,  and  was  precipitated  as  sul- 
phate on  acidulating.  The  addition  of  strong  alkali  was  found  to 
be  necessary  to  ensure  the  complete  precipitation  of  the  zinc.  The 
results  were  not  very  satisfactory,  the  following  figures  showing  a 
variation  of  ±  6  per  cent  or  more. 

Table  No.  9.    Results  of  Method  No.  4  (&). 


Contents  of  Solution  Tested. 

ago, 

by  voL 
added. 

C.SO. 
required. 

CuS04 
per  100  c.c. 
of  ferrocy. 

Percentage 
results. 
32.7  =  100 

K«FeCye. 

KCy 

0.3T3* 

KCyS 
0.3* 

K& 

0.422 

c.c. 

c.c. 

C.C. 

c.c. 

c.c. 

C.C. 

c.c. 

25 

20 

25 

20 

10 

8.7 

84.8 

106.4 

30 

25 

20 

30 

10 

9.4 

31.3 

95.7 

50 

10 

10 

25 

10 

15.7 

31.4 

96.0 

50 

10 

10 

10 

10 

16.5 

33.0 

100.9 

50 

15 

5 

20 

10 

16.6 

33.2 

101.5 

METHOD  No.  5. 
Conversion  into  Ferricyanide  and  Titration  with  Copper  Sulphate. 

In  the  method  described  by  F.  Hurter,  the  solution  is  oxidized 
by  bleaching  powder,  warmed  to  drive  off  excess  of  chlorine,  and  the 
resulting  liquid  titrated  with  copper  nitrate  using  ferrous  sulphate 
as  external  indicator. 

This  process  was  slightly  modified  in  both  the  methods  given 
below. 

It  was  not  found  advisable  to  heat  the  solution,  as  a  blue  precipi- 
tate formed  when  the  liquid  was  heated  slightly  above  140°  F.,  and 
in  this  case  the  results  were  too  low.  As  heating  at  this  tempera- 
ture did  not  readily  remove  the  excess  of  chlorine,  a  brisk  current 
of  air  was  aspirated  through  the  liquid  for  about  ten  minutes. 

In  the  second  modification,  permanganate  was  used  instead  of 
bleaching  powder  as  the  oxidizing  agent. 

Method  No.  5  (a).  Treatment  of  the  Acidulated  Solution  with 
Bleaching  Powder,  and  Titration  with  Copper  Sulphate  and  Fer- 
rous Indicator. — The  solution  to  be  tested  (containing  ferrocyanide, 
cyanide,  thiocyanide  and  zinc  double  cyanide)  was  acidulated  by  the 
addition  of  10  c.c.  25  per  cent  sulphuric  acid,  and  a  strong  solution 
of  bleaching  powder  (containing  from  1  to  2.5  per  cent  of  available 
chlorine)  was  added  in  the  cold  until  a  drop  of  the  mixture  no  longer 


154 


CHEMISTRY  OF   CYANIDE   SOLUTIONS. 


showed  any  blue  color  with  dilute  ferric  chloride  in  spots  on  a  por- 
celain plate.  In  presence  of  much  zinc  a  precipitate  formed,  which, 
however,  redissolved  in  excess  of  bleaching  powder.  Air  was  then 
aspirated  through  the  solution  by  placing  it  in  a  flask,  with  a  tube 
connected  with  a  large  jar  filled  with  water,  so  arranged  that  by 
siphoning  the  water  from  the  jar  a  brisk  current  of  air  was  drawn 
through  the  liquid  in  the  flask  for  about  10  minutes.  By  this 
means  the  excess  of  chlorine  was  practically  removed.  The  pres- 
ence of  a  moderate  excess  of  chlorine  did  not  appear  to  affect  the  re- 
sult materially,  but  the  cyanogen  chloride  attacks  the  eyes  and  ren- 
ders the  manipulation  very  unpleasant,  so  that  it  is  advisable  to 
remove  it. 

The  liquid  was  then  titrated  with  copper  sulphate  (1  per  cent 
CuS04-5H20)  using  ferrous-ammonium  sulphate  in  spots  on  a 
porcelain  plate  as  external  indicator,  until  a  drop  of  the  liquid  taken 
out  on  a  glass  rod  no  longer  gave  a  blue  color  but  produced  a  slight 
brownish  or  purplish  tinge.  The  reaction  was  considered  to  be  com- 
plete when  the  spots  became  momentarily  blue  on  mixing,  but 
showed  no  trace  of  blue  after  standing  for  a  few  seconds.  With 
practice  this  point  could  be  determined  within  about  0.2  c.c.  of  the 
copper  solution. 

Experiments  in  which  the  liquid  was  warmed  to  140°  F.  before 
aspirating  gave  less  satisfactory  results. 


Table  No.  10.    Results  of  Method  No.  5  (a). 

F.  G. 

Cyanide                 KCy          0.192%  0.105% 

Zinc  double  Cy    'K2ZnCy4   0.076%  0.057% 

Thiocyanate          KCyS        0.03  %  0.06  % 

Ten  c.c.  of  sulphuric  acid  (25%  by  volume)  added  in  each  test. 

(»)     Results  with  Aspiration. 


Model 
Solutions 


Perrocyanide 
0.422* 

Model 
Solution  P 
taken. 

Bleaching 
powder 
solution 
(about  If) 

CuS04 
required. 

CuSO4 
per  100  c.c. 
of  ferrocy. 

Percentage 
results. 
29.6=100 

chlorine. 

O.C. 

c.c. 

c.c. 

C.C. 

C.C. 

15 

50 

80 

4.25 

28.8 

95.6 

20 

50 

30 

5.6 

28. 

94  6 

25 

80 

50 
50 

80 
50 

7.6 
0.8 

80.4 
81. 

102.7 
104.7 

40 

50 

50 

11.85 

29.6 

100 

50 

50 

40 

15 

80. 

101.4 

CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

(it)     Results  without  Aspiration. 


155 


Ferrocyanide 
.422% 

c.c. 

Model 
Solution  G. 
taken, 
c.c. 

Bleaching 
Powder 
2.55%  Cl. 

C.C. 

CuSO« 
required. 

C.C. 

CuSO* 
per  100  c.c. 

C.C. 

Percentage 
results. 
31.2  =  100 

20 
25 
25 
40 
50 

50 
25 
50 
50 
25 

10 
10 
20 
15 
10 

6.2 
7.8 
7.7 
12.75 
15.6 

31. 
31.2 
30.8 
31.8 
31.2 

99.4 
100. 
98.7 
101.9 
100. 

The  first  set  of  tests  give  results  within  about  ±  5  per  cent  of  the 
theoretical  number,  while  the  second  agree  within  less  than  ±  2  per 
cent.  It  must  be  remarked,  however,  that  the  standard  of  the  copper 
solution  was  different  in  the  two  series. 

Experiments  made  with  widely  varying  amounts  of  KCy, 
K2ZnCy4  and  KCyS  gave  very  divergent  results;  nevertheless,  the 
method  is  so  rapid,  as  compared  with  any  process  depending  on  the 
complete  decomposition  of  the  cyanogen  compounds,  that  it  may 
very  possibly  be  found  of  use  in  practice,  and  if  the  copper  solution 
be  standardized  on  a  liquid  of  somewhat  similar  composition  to  that 
which  is  to  be  examined,  the  results  are  sufficiently  accurate.  If 
standardized  on  pure  ferrocyanide,  the  amount  of  copper  required 
is  considerably  higher  than  that  necessary  with  an  impure  solution 
containing  the  same  amount  of  ferrocyanide.  Thus  in  the  last 
series,  50  c.c.  of  N/100  ferrocyanide  with  10  c.c.  of  the  bleaching 
powder,  required  32.5  c.c.  instead  of  31.2,  corresponding  to  a  differ- 
ence of  over  4  per  cent. 

Method  No.  5  (6).  Treatment  of  the  Acidulated  Solution  with 
Permanganate,  and  Titration  of  Liquid  with  Copper  Sulphate  and 
Ferrous  Indicator. — After  acidulating  with,  say  10  c.c.  of  25  per 
cent  sulphuric  acid,  permanganate  solution,  generally  about  decinor- 
mal,  was  run  in  until  a  slight  coloration  remained  permanent,  indi- 
cating that  all  reducing  agents  were  oxidized  and  the  whole  of  the 
ferrocyanide  converted  into  ferricyanide.  The  liquid  was  then 
titrated  with  copper  sulphate  and  ferrous  ammonium  indicator  as 
in  method  5  (a). 

When  very  large  quantities  of  reducing  agents  were  present,  so 
that  much  permanganate  was  required,  a  dark  brown  precipitate 
occurred  which  interfered  somewhat  with  the  test ;  in  such  cases  less 
than  the  theoretical  amount  of  copper  sulphate  was  required. 

Zinc  appeared  to  interfere  considerably,  rendering  the  results 
much  too  low.  Some  results  are  given  below  in  which  the  zinc  was 


156 


CHEMISTRY   OF   CYANIDE  SOLUTIONS. 


removed  by  precipitation  as  sulphide  after  making  strongly  alka- 
line and  heating.  The  filtrate  was  then  acidulated;  oxidized  by 
permanganate  and  titrated  with  copper  sulphate,  as  above. 

Table  No.  11.    Results  of  Method  No.  5  (&)  in  Solutions  Free 
from  Zinc. 


f 
Model  Solution 

[ 


-3H20 


Na2S 


0.013% 


Varying  amounts  of  cyanide  were  added  as  shown  below: 

Ten  c.c.  of  25  per  cent  sulphuric  acid  were  added  in  each  test, 
and  the  permanganate  solution  (about  N/10)  was  run  slightly  in 
excess  of  the  amount  required  to  give  a  distinct  red  color. 

100  c.c.  of  mixture  H  =  60  c.c.  N/100  ferrocyanide. 

The  results  appear  to  be  unaffected  by  variation  in  the  amount 
of  cyanide,  but  the  copper  solution  should  be  standardized  on  a 
mixture  of  similar  composition  to  the  liquid  to  be  tested. 


Table  No.  11. 


Volume  of 
model  solution 
H  taken. 

Cyanide 

0.672* 
KCy  added. 

KMn04 

(TOf- 

CuS04 
required. 

CuS04 
per  100  c.c. 
N/100  ferro. 

Percentage 
results. 
31.4=100 

c.c. 

c.c. 

c.c. 

C.C. 

c.c. 

50 

23 

9.4 

81.3 

99.7 

60 

10 

30 

11.3 

31.4 

100 

50 

15 

28 

9.4 

81.3 

99.7 

50 

15 

80 

9.6 

32 

1C1.9 

50 

20 

25 

9.2 

30.7 

97.8 

50 

80 

25 

9.3 

81 

98.7 

50 

80 

22 

9.4 

31.3 

99.7 

50 

85 

25 

9.7 

82.3 

102.9 

50 

40 

21 

9.5 

81.7 

101 

These  results  show  a  variation  of  ±  3  per  cent  from  the  theoret- 
ical value.  Similar  tests  with  varying  amounts  of  ferrocyanide 
gave  a  variation  not  exceeding  9  per  cent. 

Table  No.  12.  Tests  by  Method  No.  5  (b),  in  which  zinc  was 
originally  present,  but  was  removed  by  preliminary  treatment  with 
sodium  sulphide. 

The  following  test-solutions  were  prepared : 


CHEMISTRY    OF    CYANIDE    SOLUTIONS. 


157 


No.  of 
Test  Solution. 

Ferrocyanide 
K«FeCye.3H,0 

K,ZnCy4 

KCyS 

KCy 

i 

% 

t 

* 

1 

0.0428 

0.057 

0.015 

0.10 

2 

0.0638 

0.038 

0.06 

0.05 

3 

0.0844 

0.095 

0.045 

0.05 

4 

0.1055 

0.057 

0.03 

0.125 

5 

0.1688 

0.038 

0.045 

0.10 

6 

0.2110 

0.076 

0.03 

0.075 

The  results  of  tests  with  these  solutions  are  shown  below : 


Table  No.  12. 


No.  of 
tost 

Na-S 
0.185X 

NaOH 

10* 

Added  to  Filtrate. 

CuSO4 
per  100  c.c. 

Percentage 
results. 

solution. 

added. 

added. 

KMnO 

H,S04 

CuS04 

N/100 

30=  100 

3* 

25* 

required. 

ferrocy. 

C.C. 

C.C. 

C.C. 

C.C. 

C.C. 

c.c. 

1 

20 

10 

10 

20 

3 

30 

100 

2 

10 

10 

3.7 

80 

4.5 

30 

100 

3 

25 

10 

4.5 

20 

6.2 

81 

103.3 

4 

15 

10 

3 

20 

7.2 

28.8 

96 

5 

15 

10 

4 

20 

11.95 

29.9 

99.7 

6 

25 

10 

5.8 

20 

15.05 

80.1 

100.3 

These  numbers  show  a  variation  of  about  ±  4  per  cent. 

METHOD  No.  6.     (MULLER'S  METHOD.) 
Titration  with  Zinc  Sulphate. 

A  measured  quantity  of  the  solution  to  be  tested  was  acidulated 
with  sulphuric  acid,  and  titrated  with  zinc  sulphate  (0.25  per  cent 
Zn) ;  the  end-point  was  determined  as  in  Bohlig's  method  (No.  4), 
by  placing  a  drop  of  the  liquid  on  white  filter  paper,  the  exact  fin- 
ishing point  being  found  by  means  of  a  dilute  solution  of  ferric 
chloride.  When  this  ceased  to  show  the  slightest  blue  tint,  the  re- 
action was  considered  complete. 

The  method  was  found  to  be  useless  in  presence  of  zinc  double 
cyanide,  and  in  absence  of  zinc  did  not  give  results  even  approxi- 
mately proportional  to  the  amount  of  ferrocyanide  present,  although 
duplicate  tests  on  solutions  containing  the  same  amount  of  ferro- 
cyanide generally  agreed  very  fairly.  This  variation  in  the  re- 
sults is  possibly  due  to  the  existence  of  several  ferrocyanides  of 
zinc,  formed  under  different  conditions.  The  results  were  not  ap- 
preciably affected  by  variation  in  the  amounts  of  cyanide  or  thio- 
cyanate. 


158 


CHEMISTRY   OF   CYANIDE   SOLUTIONS. 


Table  No.  13.    Results  of  Method  No.  6,  in  Absence  of  Zinc  Double 
Cyanide. 


Mixture  Tested. 

by  volume 

ZnS04 
(.25*  Zn) 
required. 

ZnS04 
ferrocyanide 
per  100  c.c. 
of  N/100 
ferro. 

Percentage 
results. 
39=100 

N/100  ferro- 
cyanide. 

0^* 

*$? 

C.C. 

c.c. 

C.C. 

C.C. 

c.c. 

c.c. 

25 

4 

10.7 

42.8 

109.8 

50 



4 

19.4 

38.8 

99.5 

50 



19.95 

39.9 

102.3 

50 

10 

_ 

19.3 

38.6 

99 

50 

20 



19.2 

38.4 

98.5 

50 

50 

_ 

19.6 

89.2 

100.5 

50 

10 

19.2 

38.4 

98.5 

50 



20 

19.2 

88.4 

98.5 

50 



25 

19.05 

88.1 

97.7 

50 



50 

19.8 

89.6 

101.5 

50 

25 

25 

19.75 

89.5 

101.3 

These  results  (excluding  the  first,  in  which  a  different  amount 
of  ferrocyanide  was  used)  show  a  divergence  of  not  more  than 
±  2.5  per  cent.  The  method,  however,  cannot  be  recommended, 
owing  to  the  irregularities  occurring  with  varying  amounts  of 
ferrocyanides. 

When  zinc  double  cyanide  is  present,  a  partial  precipitation  of 
the  ferrocyanide  as  Zn2FeCy6  occurs  on  acidulating  with  sulphuric 
acid,  hence  only  the  portion  still  remaining  in  solution  is  indicated 
on  titrating  with  zinc  sulphate. 

METHOD  No.  7. 
Green's  Method. 

This  method  is  described  incidentally  in  a  system  of  tests  de- 
signed principally  for  the  estimation  of  cyanide,  alkali  and  zinc,  and 
published  by  Leonard  M.  Green  (Proceedings  Inst.  Min.  and  Met., 
Oct.,  1901).  It  involves  the  execution  of  the  other  tests.  Some  of 
these  latter  are  useful  and  give  results  agreeing  satisfactorily  with 
theory,  but  the  method  of  determining  ferrocyanide  is  too  involved 
and  indirect  to  be  of  much  practical  value.  The  results  showed 
only  a  very  rough  approximation. 

There  are  four  operations,  as  follows: 

(1)  Total  cyanide  is  determined  by  taking  n  c.c.  of  the  solution 
to  be  tested,  adding  5  c.c.  of  strongly  alkaline  potassium  iodide 
solution  (say,  4  per  cent  NaOH  and  1  per  cent  KI),  titrating  with 


CHEMISTRY  OF  CYANIDE  SOLUTION'S. 


159 


silver  nitrate  until  a  distinct  yellow  turbidity  remains  permanent 
on  shaking.    Eesult  =  T. 

(2)  Take  again  n  c.c.  of  the  solution  to  be  tested,  add  one  or  two 
c.c.  of  neutral  potassium  chromate,  and  titrate  till  the  liquid  re- 
mains permanently  red  on  shaking.     (Where  much  free  alkali  is 
present  this  must  be  carefully  neutralized  before  making  this  test.) 
Eesult  =  N. 

(3)  Take  n  c.c.  of  the  solution  to  be  tested.    Add  2T  c.c.  of 
silver  nitrate,  then  phenol  phthalein  and  sodium  carbonate  till  dis- 
tinctly red.     Then  exactly  neutralize  by  adding  nitric  acid  drop  by 
drop,  shaking  and  allowing  to  settle,  until  the  liquid  becomes  quite 
colorless.     Then  add  ferrocyanide  in  excess  (say,  about  25  c.c.  of 
N/10  solution),  which  causes  the  solution  to  become  alkaline  again. 
Titrate  with  N/10  nitric  acid  until  the  color  is  discharged,  leaving 
a  slightly  yellowish  liquid.     The  nitric  acid  must  be  run  in  slowly, 
with  constant  agitation  toward  the  end.    Eesult  of  last  titration 
=  A. 

(4)  Take  n  c.c.  of  the  solution  to  be  tested,  add  N  c.c.  of  silver 
nitrate,  and  proceed  exactly  as  in  (3).    Eesult  =  B  (proportioned 
to  zinc  present) . 

The  quantity  B  —  A  should  be  proportional  to  the  ferrocyanide 
in  the  original  solution. 

Table  No.  14.    Results  of  Method  No.  7. 
25  c.c.  NyiO  ferrocyanide  added  during  each  test. 

K4FeCy6-3H20  0.1% 

KCy  0.2% 

KCyS  0.1% 
K2ZnCy4 


Model  Solution  J 


Results  of  Tests. 

Vol.  of 

model 

B—  A 

B  —  A 

B—  A 

solution  J 
taken. 

CD 

9 

5 

B? 

per  100 
c.c. 

Percentage 
results. 

AgNO, 

AgNO, 

HNO, 

HNO, 

OfJ 

5.93  =  100 

C.C. 

c.c. 

c.c. 

C.C. 

c.c. 

c.c. 

15 

18.5 

33.75 

8.05 

8.75 

0.7 

4.7 

79.8 

20 

24.4 

57.8 

4.1 

5.2 

1.1 

5.5 

92.7 

25 

30.3 

73.4 

4.6 

5.85 

1.25 

5.0 

84.3 

80 

86.45 

86.9 

5.65 

7.4 

1.75 

5.9 

99.5 

40 

48.9 

116.35 

7.15 

9.55 

2.4 

60 

101.9 

50 

61 

143.4 

8.5 

11.45 

2.95 

5.9 

99.5 

160  CHEMISTRY    OF    CYANIDE    SOLUTIONS. 

In  the  first  three  tests  the  quantities  of  nitric  acid  which  have  to 
be  determined  are  probably  too  small  for  accurate  measurements 
under  the  conditions  of  the  test. 


CONCLUDING  EEMAEES. 

Of  the  different  methods  examined,  that  described  under  1  (6) 
gave  the  most  closely  concordant  results,  and  with  care  could  no 
doubt  be  applied  with  great  accuracy  for  the  determination  of  fer- 
rocyanides  under  ordinary  conditions.  It  is,  however,  decidedly 
too  long  for  general  use  in  cases  where  determinations  have  to  be 
made  at  frequent  intervals. 

For  rapid  approximate  results,  Hurter's  method  (No.  5  (a)  ) 
with  the  modifications  herein  described,  appears  to  be  the  most  serv- 
iceable. It  is,  however,  absolutely  essential  that  the  standard  cop- 
per solution  should  be  standardized  on  a  liquid  (containing  an  ex- 
actly known  quantity  of  ferrocyanide),  and  having  other  ingredients 
approximately  in  the  proportions  which  may  be  expected  to  occur 
in  the  liquid  to  be  examined. 

The  investigation  is  far  from  complete,  and  many  tests  were  made 
on  various  minor  points  to  which  no  reference  has  been  possible 
in  this  paper.  In  the  results  which  are  here  presented,  however, 
every  effort  has  been  made  to  eliminate  the  personal  element ;  when- 
ever possible,  the  solutions  were  prepared  by  one  operator,  and  tested 
by  another  who  was  not  previously  informed  of  their  composition. 
It  is  hoped,  therefore,  that  the  data  which  have  been  obtained  may 
serve  as  a  useful  basis  for  further  researches. 


INDEX 


INDEX. 


Active  cyanogen  compounds.  .2,      4 
Active  haloids,  estimation  of...   100 

Adair,  Alfred,  cited 76 

Alkaline    constituents 2,     58 

action    toward    alkaline    hy- 
drates        59 

action  toward  alkaline  mono- 
carbonates    59 

action    toward    alkaline    sul- 
phides       60 

action   toward   alkaline  zinc- 


action   toward   ammonia 60 

action  toward  bicarbonates..     59 
action     toward     simple    cya- 
nides        58 

action  toward  zinc  cyanides. .     60 
Alkaline  earths,   estimation  of.  137 

Alkaline  iodide  indicator 10 

Alkaline  metals,   estimation  of.  137 
Alkalis,  test  illustrating  the  in- 
fluence  of    11 

Ammonia,    estimation   of 68 

Ash,    determination   of 139 

Auxiliary  agents 2,     95 

Available    cyanide,     estimation 

of 54,     56 

Base    metals    2,123 

Bettel,    W.,   cited.. 23,   30,   35, 

53,  56,  68,  72,  103 
Bicarbonates,    estimation   of.66,     67 

Brown,  E.  O.,  cited 134 

Carbonates,    estimation   of.  .66,     67 

Chlorides,   estimation  of 108 

tests  illustrating  the  influence 

of    16 

Christy,  S.  B.,  cited 116 

Colorimetric    method    for    esti- 
mation  of   sulphides 92 

for     estimation     of    thiocya- 

nates 89 

Copper,  estimation  of 134,  135 

Crosse,  A.  F.,  cited 42,  65, 

97,  99,  126 

Cyanates  and  isocyanates,  esti- 
mation of 104 

Cyanide    solutions,    ingredients 
ol    2 


Cyanogen   bromide,    estimation 

of 100 

Cyanogen    in    compound     cya- 
nides, estimation  of 42 

Deniges,    G.,    cited 8,    9,    10 

Durant,    H.    T.,    cited 120 

Ellis,    Charles   J.,    cited 26,     36 

Ferricyanides,     estimation     of, 

102,  103 
Ferrocyanides,     estimation     of, 

73,  74,  75,  78,  79,  82,  83,     85 
test  illustrating  the  influence       m 

of    19 

Free    cyanide,     estimation     of, 

4,  26,  28,  31,     32 

Gasch,  R.,  cited 83 

Gasometric  method  for  estima- 
tion of  oxygen 96 

Gold,  estimation  of . . .  .114,  118, 

119,  120 
Gold  and  silver,  estimation  of.  114 

Goyder,  G.  A.,  cited 35,  36,     37 

Gravimetric     determination     of 

total  cyanogen    49 

Green,  L.  M.,  cited.  .34,  49,  64,  132 

Green's  method    158 

Haloids,  active 100 

Hannay,  J.   B.,  cited 31 

Herting,    O.,   cited 107,  108 

Hurter,  F.,  cited 82 

Hydrates,  estimation  of 66,     67 

Hydrocyanic     acid,     estimation 

of    52,     53 

Inactive  bodies   2,  104 

Iodine  absorbents,  tests  for 98 

Iron,  estimation  of 137 

James,  Alfred,  cited 116 

Knublauch,    O.,   cited 80 

Knublauch's    method 80,     81 

Kraut,   K.,   cited 11 

Lead  salts  in  estimation  of  sul- 
phides     90,     91 

Lenssen,  E.,  cited 102 

Leybold,  W.,  cited 85 

Liebig's   method,  or  estimation 

of  free  cyanide   4,  78,     23 

Loevy,  J.,  cited 92 

Longmaid,  John,  cited 33 


164 


INDEX. 


Maclanrin,  J.  8.,  cited 55 

McArthur,  J.  T.,  cited 8,    26 

Mellor,  J.   W.,  cited 107,  112 

Mtiller,  JM  cited 83 

Nitrates,    estimation    of 93,  112 

Noble  metals 2,  114 

Organic  matter,   estimation  of.     72 
Oxidizable  organic  matter,  esti- 
mation of 72 

Oxygen,  estimation  of.  .95,  96,    97 

test  for 98 

Peroxides,   estimation  of 101 

Protective  alkali,  definition  of.    61 

estimation  of 63,   64,     65 

Reducing    agents    2,     70 

Reducing  power,   definition   of.     72 

estimation    of 70 

Rose,    H.,   cited 51 

Rose,  T.,  Kirke,  cited 24 

Sharwood,  W.  J.,  cited.  .11,  24, 

27,  35,  85,  113 

Silicates,   estimation   of 113 

Silver,  estimation  of 114,  120 

Sodium  nitroprusside,   prepara- 
tion of   92 

Sulphates,  estimation  of 112 

Sulphides,  estimation  of.. 90,  92,    93 


PAQB 

Sulphocyanides  (see  thiocyanates). 

Suspended  matter 2,  138 

Sutton,  cited 53 

Tcherniac,   J.,    cited 75 

Thiel,  A.,  cited 88 

Thiocyanates,  estimation  of..87,    89 
tests    illustrating    the    influ- 
ence of  23 

Thorpe,  cited 77 

Thresh's  method  for  estimation 

of  oxygen    97 

Total  alkali,  definition  of 61 

estimation    of 62 

Total    cyanide,    estimation    of, 

33,  34,  37,  38,  39,    40 
Total  cyanogen,  estimation   of, 

47,  48,     49 
Total  solids,  estimation  of.. 138, 

139,  140,  143 

Victor,  E.,  cited 106 

Vielhaber's  method   48 

Virgoe,  W.  H.,  cited 38 

Watson,    Henry,   cited 118 

Weith,    cited 51 

Whitby,  cited   117 

Zaloziecki,   R.,  cited 84 

Zinc,  estimation  of 123,  124, 

125,  126,  127,  128 


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